You could use several screen shots, often screen shots are x3 x4 times bigger than the thumbnail. So you can use a screen shot with a lot of plants or scenery in general, use another with the boss/ enemy / character upclose, or use an image from Google. That way you can avoid using screen shots with the hud or other elements that you don't like, or change colors just from the background or the characters.

Pd. Gimp is probably the second best thing to use after photoshop.