I don't know if this is what you want, but if you're not after anything very fancy, here's what I did to make an icon for my first level: I made a screenshot from a screen of my level, opened that screenshot in my graphics program (I have something slightly, but not much better than MS Pain - but for this, you don't really need anything that Paint can't do), made a 30x30 pixel selection from that, and saved that as a png file - and that's my icon. (I'm currently using that as my user picture here as well.)