Yes, it is possible to make a sig using paint but it is very difficult. Or atleast to make a good one. Take it from me. Just pester people constantly so they make one for you.
Here are couple examples of my past endeavors in Paint sigs. Laugh away. The first I was actually happy with. but it took me an hour just to take out all the white bumps that were at the edges of the character models. The last ones I absolutely hate as 1) It is basically me ripping off an old member of the site 2) It was made in the time that I really really sucked at role-playing 3) I was using a character model that I now see as entriely stupid.
Yeah, I forgot to mention that paint is horrible for editing images witht he actual painting functions. I just use it to put pictures together, put text on them, and to color in background space.
I didn't do the clone trooper I just found a pic of the clone trooper took all the white armor and darkened it, added the red, and changed the visor color. Then put in the name.