I tend to disagree with Zubaz to some extent, I think this site is equally about wallpapers as it is about any other customization tool. A wallpaper can be a massive part of your theme as is evidenced in the screenshot section, where, if you don't get the right wallpaper with your skins it can just look totally wrong. Same goes for DesktopX themes.
Uploading wallpapers here can be a little frustrating. It's hard to know whether you've created something that many people will like until you actually put it on the site and see what kind of reception it gets. I've been making walls for years now and have 800,000 downloads in that section, but I still don't know what people like. (Well, if you write vista on it you will double your download count)
All I would say to you is, try not to use the standard Photoshop filters too much, if at all. Choose your colors very carefully, use a color wheel if necessary. And, as someone else said, search the web for good tutorials, follow them, and then put your own spin on things.
Don't get too downhearted, I used to get work rejected constantly when I first started, but I persevered, and now I'm the number 3 wall artist on here.
Patience Grasshopper!! haha.
Good luck...... don't give up.
