Check out this screenshot (Heroes 3 + Photoshop).

Arseny, look how easy it is. You launch the game, create a beautiful moment, and simply press the **Print Screen** button once (it's right after F12). Then, open Paint, select the "draw rectangle" button, right-click, choose "paste," and save the image. And then, use Photoshop!
This is the simplest method; there are also ACDsee, SnagIt, and others. I used to take 10 screenshots and then cut out the parts I needed.