Writing a full tutorial on this kind of thing is hard because every image is different, so you can't do a 'this works for most things' kind of tutorial. However! The best tools for this are usually Brightness & Contrast meets Levels.
My computer with my programs are in the shop, so I used pixlr to mess a bit. Under Pixlr, these tools are under adjustment. I believe the path in Photoshop is Edit > Levels and Edit > Brightness & Contrast, I'm not at all sure about Gimp.
If you open Levels first, you should see a graph looking thing and three arrows at the bottom. Grab the far right arrow, it may be white depending on your program/version. If you drag this left, it will increase the white in the image, thus making it brighter. The middle one can be played with to try and keep things from being too washed out. You really have to play with it, it's an eyeballing thing.
Then you can go to Brightness and Contrast. Brightness will lighten the whole picture, and can really fade it out. Get it to a brightness you like (pull the arrow right) then you can mess with the contrast to help any bleach-outs.
Lastly, you might mess with Hue & Saturation or Color Balance. This image, for example, turned very very yellow with the adjustments, playing around with those two or a combination of those two can take it back more towards 'normal' colors.
I will note that not every image can be 'saved,' but this is a good way to give it a fighting chance.