Ok, first point. The thumbnails rarely do anything justice, but they can only see the full thing if they click on the thumbnail. The way you convince people to have a sneak peak is to have an interesting comment or info on the making in the comment box of the wallpaper when you submit.

Second point. It's all subjective, direct this question to Jafo or paxx and they will be able to answer in greater detail. Check this offical (but rough) set of guidelines: https://www.wincustomize.com/guidelines.asp

Third point, you don't need WinXP to create your logon themes. All you need is LogonStudio, downloadable from https://www.stardock.com/products/logonstudio/ However, it is good to have a friend who does have WinXP install logon studio and test your logon before you submit.