Interesting poses is the main thing. And by interesting, I don't mean crazy awkward positions, which is what quite a few screen shot people think when someone says interesting poses, lol. Just avoid taking more than 1-2 front portrait shots of the subject matter. (But then again there are a few, not all, but a few artists who won't do anything unless it's a front facing screen due to lack of skill, so this advice is a tad bittersweet.)
Also, something that's not busy. Most artists on here repaint, and if they just edit, they still have some ability to paint on extra things instead of having to work around it on the image.
I don't know, things might be different now. I haven't 'edited' an imvu screenshot in years.