1. find part of the most interesting part of the picture, and use that part.
2. Light or dark..eh..contrast and detail, or implied detail
3. colors..in fact, bright colors..;)
4. size matters not..;) smaller for less detailed, larger for more detailed
5. If you're new, noone's paying attention..;) the picture does more than the name..;)
6. no n*dity, anyone who has flagged not to see it won't see the thumbnail (or even the entry). If you must, well, you can do what I saw once, use say, the curve of the elbow to suggest another curve (the guy got a lot of hits over that..;)
7. Amount of details would again depend on the size of the thumbnail.
8. POV- and just a general notation. Have the eyes looking at something, preferably the viewer, will get more attention
9. Title- for some odd reason, punctuation seems to work..don't know why. Sometimes a pun, a risque double entendre`, etc., depending. Pictures with numbers after them don't grab you the way 'Space monsters battle the bikini squad' would..;)
   I'm a mediochre (sorta red..;) artist, but as I follow these rules, my hit counts have gone up..hope this helps.

I wish I'd said that.. The Staircase Wit

anahl nathrak uth vas betude doth yel dyenvey..;)