I am not an icon expert. But I do have some basic knowledge of a few simple things you can do to make your icons look better, especially when working with screencaps. Some of this advice is specific to photographs, as opposed to illustrations. But some is applicable to any icon.
This is a compilation of things I learned from other icon tutorials, from noodling around, and from the designers I work with.
I am using Photoshop CE, but these techniques are very basic and so should work pretty much the same back several versions.
As an example, I'll make an icon from one of the PoA trading card scans up at The Leaky Cauldron.
Here's the original image:
Cropping & Resizing
This is the crop tool:
To crop an exact square, hold the shift key down as you go. In my version of Photoshop, up in the bar just below the menus, I can preset the size of the cropped area:
resolution: 72 pixels per inch
That way, I can draw the square any size I like and when I crop it, it will automatically be resized to 100x100. Otherwise, you'll need to resize manually.
NB: don't make your image 100x95 or 100x90. People will look at it and think something is a bit odd. If you don't want to use a square, go for something like 100x60 or 100x70.
Cropping the image determines the impact that it's going to have. A good rule of thumb is that it's usually more interesting if the focus of attention isn't right smack in the middle of the icon. Off to one side is good. That doesn't mean you have to leave the rest of the area blank, but the focus -- maybe the eyes -- should be off to the side.
If you want a more concrete example, put the focus -- the most interesting part -- on one of the red dots:
But I'm usually not that exact. I'm not an expert cropper, but I'm better than I used to be.
Keep in mind: you don't have to put the whole head in the frame. Crop in closer. You can use unusual cropping to bring focus on certain features or elements. Here I wanted attention to be on Kaylee's pretty smile, so I left the eyes out altogether:
Here's how I've cropped Ron. The eyes are more or less in the top right quarter of the frame and we get a good look at his poor worried expression:
Usually a screen cap isn't all that great quality, so you'll want to make some changes to the levels and maybe the colour.
The first thing I do is to make two copies of the layer. The bottom layer I lock and turn off the visibility. That's just so if I screw something up later, I can go back to it.
Next apply some adjustment layers and blending options. I don't always do these in the same order.
Now I have two identical visible layers. On the top layer, I'll change the blending. There's a drop-down menu on the layers palette that usually says "normal". There's lots of stuff under there, though, and it all changes how the layer blends with the visible layers beneath it. Try a few and see what they do. The one I use most often is Soft Light. I find that it gives an interesting definition to the photo without making it look too much like a special effect.
I don't always use blending options. But they can be cool.
Here's Ron with the top layer set to Soft Light:
Adjustment layers are one of the most useful things in Photoshop. You can easily change things like level, contrast, colour, etc, but since those changes are on their own layer, you can remove them later or edit them further.
The thing you'll most likely want to change is the levels. Screenshots are often rather dark and murky, so we want to brighten things up.
Select the top layer. From the bottom of the layers palette, click on the adjustment layer icon: and choose Levels.
You'll get a window with sliders, like this:
They control the light levels. I never change the output levels; I only use the input level sliders.
On the right is the highlight slider, the white one. Usually, I pull that one up to around where the black line starts.
On the left is the shadows slider, the black one. I sometimes do the same with that -- pull it to where the black starts, to make the shadows a little deeper.
In the middle is the midtones slider, the grey one. It's best to fiddle with that one last. Usually I'll bring it to the left a bit more until I think the picture looks better.
Click the preview box on and off and you'll see what a difference it makes. When you're happy, just click OK.
When I was done with Ron, my sliders looked like this:
And here's Ron with the level adjustment applied:
Play around with some of the other adjustment layers too. I often use them to adjust the colours when I think the skin is too red or too green, for example. As well, if the picture is a bit washed out, add a bit of saturation to it.
Here's Ron with a Hue/Saturation layer with +15 to saturation:
Most icons have some sort of caption. Choose your text, choose your font, and add it. It often looks good to use a colour for the text that's already in the photo. I use the eyedropper tool to pick up a colour -- light or dark as appropriate -- then use the colour picker to change it a bit, while still keeping it in the same range. (Usually white or black will look good too.)
Most fonts can be set to whatever size you like. On the character palette, look for the Anti-Aliasing menu at the bottom. It gives you a choice between "none", "sharp", "crisp", "smooth", or "strong". Normally, I use strong since it gives the type a bit of a boost in intensity and can make it easier to read. But use whatever you think looks best for your type.
NB: Pixel fonts, also called bitmap fonts, like Silkscreen, or 04b, are intended to be used only at one size (often 8px) and should have the anti-aliasing set to "none". They are often much more readable at small sizes than normal fonts.
Text on an icon can be hard to read, so to set it off, you might want to add a Layer Effect such as a drop shadow, outer glow, or stroke (outline). The Layer Effect menu is on the bottom of the layers palette and looks like this: Try a few and see what looks best on your type.
For Ron, I used the font LainieDaySH. I picked up the colour from Ron's face and lightened it so it was an off-white. The anti-aliasing is set to "strong". I didn't add a shadow or a glow -- the text was readable without them.
I like borders. I think they make an icon look more finished -- it gives it closure. There are lots of different things you can do, but the easiest is just a simple stroke around the edge.
Add a new layer at the very top, then select all. From the Edit menu, choose Stroke. Set the width. I often use 1px or 2px.
Use the eyedropper to pick up a colour you want, just like with the type. I often use black or a dark colour from the photo, but sometimes a lighter colour can look good too.
Location should normally be set to "Inside". Click OK and there you go.
I added a 1px black border to Ron. Since the photo was already so dark around the edges, it doesn't stand out. But I think it makes a difference.
This is a very important step. Unless you are deliberately going for a blurred or grainy look, sharpening will make the photo look much more professional.
This is a little bit complicated. You could actually do this part a lot earlier, before you add text and a border and it would be easier. I usually do it last because I prefer to export the sharpened icon, but then not save the sharpened version of the Photoshop file. That way, I can make adjustments later on to the photo layers if I want to tweak the icon.
First, save your file. Then in the layers palette, select one of your visible photo layers. Link the other photo and adjustment layers to it by clicking in the blank spot next to the eye. That will add a little link icon. Don't link the text or border layers.
At the top of the layers palette, on the right side, click on the arrow. A menu will drop down. Choose Merge Linked. They will all collapse into one layer.
Make sure that layer is selected. From the Filters menu, go to Sharpen and over to Unsharp Mask. Amount should usually be 100%, Radius 0.4, and Threshold at 0 or 1. Click OK.
(Note: there's actually a formula for determining the Radius in Unsharp Mask: pixels per inch/200. When you're working with files for screen viewing, you should be at 72 ppi. So, .36, which Photoshop rounds to .4.)
I ran into a bit of a problem with Ron here, because this is a scan of a trading card, not a screen shot. It's pretty grainy and sharpening too much can make it look worse, not better. (I should probably have tried sharpening the file when I began. It's too late now. *g*) I set the Amount to 50% and that looked not bad. But maybe the non-sharpened version was better.
With a screenshot or a photo, though, sharpening often makes quite a difference.
Export your file as a jpeg. For icons, I often use a fairly high quality, like 60 or 80. For website graphics, I usually stick with 40 or lower.
Here's Ron with just text added, and then with all the adjustments and border:
I welcome your input and/or corrections. :)