OK, I don't know all of them, but gimp is great, and I use Paint.net myself (at work). It misses the soft-edged brushes, which sucks, but it has great layering and blending capabilities, so you should be able to do a lot with that.
In Paint.net, once you have the icon cropped and resized, what you do is:
1) you duplicate your base layer and then
2) apply an adjustment to it. E.g. Levels. The sliders there are vertical (not horizontal like in PS). Normally, caps are always too dark, so in the original image (on the left), the peaks will all be clustered at the bottom, and what you want to do is spread out the peaks on the right side over the whole spectrum. So you slide them up to make it lighter.
Don't hesitate to overdo it here, it's just a copy, and you can lower its opacity again in the next step.
You can also use Curves or Contrast adjustment instead of Levels, they work well, too. It's a matter of preference, whatever you like working with.
So. Then you have a lightened copy of your layer. Double click on it and then play with the blending mode and opacity. Additive and screen will give you lightened results, overlay and multiply will darken the image. If you want to avoid the colors becoming too vibrant, you can always desaturate your copied layer.
Generally, don't be afraid of having tons of layers. If you see tutorials, people often have 20 or 30 layers in their files. I usually make do with less than 10, but always at least 4 (base, sharpened copy, lighting, texture).
I hope this is more helpful, since my first comment was very Photoshop-specific.
no subject
In Paint.net, once you have the icon cropped and resized, what you do is:
1) you duplicate your base layer and then
2) apply an adjustment to it. E.g. Levels. The sliders there are vertical (not horizontal like in PS). Normally, caps are always too dark, so in the original image (on the left), the peaks will all be clustered at the bottom, and what you want to do is spread out the peaks on the right side over the whole spectrum. So you slide them up to make it lighter.
Don't hesitate to overdo it here, it's just a copy, and you can lower its opacity again in the next step.
You can also use Curves or Contrast adjustment instead of Levels, they work well, too. It's a matter of preference, whatever you like working with.
So. Then you have a lightened copy of your layer. Double click on it and then play with the blending mode and opacity. Additive and screen will give you lightened results, overlay and multiply will darken the image. If you want to avoid the colors becoming too vibrant, you can always desaturate your copied layer.
Generally, don't be afraid of having tons of layers. If you see tutorials, people often have 20 or 30 layers in their files. I usually make do with less than 10, but always at least 4 (base, sharpened copy, lighting, texture).
I hope this is more helpful, since my first comment was very Photoshop-specific.