I go for the thumbnails that have some type of layout in them... Meaning for scenic walls, everything has to be placed
in a way so it looks rather real. The colors have to look good, too. If in the thumbnail, the colors are too shocking,
too saturated, or too dull, then in the bigger image, it's probably not going to be much better. The exception
would be wallpapers with "black" backgrounds... simply because they're often starfields, and it's
hard to see the stars in the thumbnails >__<; Also, if I see something that just looks interesting in any way,
I'll click it.
As for thumbnails that turn me off... They're usually the ones that just have a character over a one-step filter
rendered background -_-;
Layouts, wallpapers, avatars,
templates, brushes, and tutorials for you ^-^;
. . . I express myself. Do you?