Also, I'm a pedant, and I love making up icon tutorials. It's so much fun!
I found the darkest cap I had:
Then I duplicated this layer twice. (Right click >> Duplicate Layer.) The top layer I set to Soft Light, the middle layer I set to Screen. The background layer I left alone. When your layer is selected, if you look up in the layer palette to where it says Mode: Normal, you can use the arrow to the right to change the layer mode, which determines how it interacts with the layers below. Here's how the image changed:
Since this was a really dark cap to start with, I'm going to duplicate the layer set on Screen. If you duplicate a layer that already has its mode changed, that will duplicate, as well, so you won't have to set it yourself. Here's with two Screen layers sandwiched between the background - which is the raw image - and a Soft Light layer on top:
So you can see that helped. I'm going to brighten it up some more, though. I don't want to do a third Screen layer; too many and the image can start getting washed out. I create a New Layer on top, and fill it with a very light creamy beige using the Paint Bucket. Any light color would work - play around with different shades and see what you get. I set the beige layer to Soft Light:
Not only does this lighten things up, it also puts some color in their cheeks; they were looking kind of ashy. Now, Gimp doesn't have adjustment layers like Photoshop does, so next I have to do something I hate: merge my layers. I never do this in Photoshop, because if I change my mind and want to go back to an earlier step, all of my layers are still there. But I couldn't figure a way around it. So go to Image >> Merge Visible Layers. Keep it as "Expanded as Necessary." Now you should be back to one single layer.
Go to Colors >> Levels. You'll see this box come up:
I'm going to change the input levels by dragging the sliders around. I would recommend experimenting with this and getting a feel for it. The lines on the left are your dark values, on the right are your light values; you can see we have a lot more darks. By moving the right and left sliders, you increase or decrease those amounts. Moving the center slider will make the overall picture lighter or darker. Here's the inputs I used, and then the result:
You can see, again, it's not a huge change, but they start adding up. Quickly, a side by side comparison of where we started, and where we are now.
Another thing I like to use is a Gradient Map. Starting with the raw screencap again, I duplicate the layer. Then, over in the tool box, I select a nice honey color for the foreground and a blue-green for the background (exact shades listed below). With my top duplicate layer selected, I go to Colors >> Map >> Gradient Map. It will look really weird. It's okay, though; you're not going to leave the mode in normal. Switch it to Soft Light. This makes for a very subtle change; I love this particular gradient map, with these colors, but it works best on a reasonably brightened image first.
Here's a different pair of colors, which I will use on darker images like this. Same method as above. Note: I usually will add the gradient map on top of the background/duplicate on Screen/duplicate on Soft Light layer sandwich, which I use on 95% of screencaps. In these two examples I didn't, just to show how much an improvement you can get even with just that one step.
That's it! Hope it's helpful. What I mostly learned in playing around with Gimp is that even though they say it's easier and more intuitive to use than Photoshop...I found it wasn't. Maybe that's just because I've been using Photoshop for seven years and Gimp for approximately forty-five minutes, but I highly recommend Photoshop.
So, um, if you want a really, really, REALLY cheap place from which to procure Photoshop, shoot me an email. All completely legal, of course.