Wallpapers

[iFid]

should they be saved in a certain format
.png or .gif or something?

[Fairy]

They should be either .jpg or .bmp :)

[Christine]

if it's a really simple wallpaper that doesn't have many colors, you can save it as a .gif. otherwise the image quality will suck so save it as a .jpg or .png
I don't recommend saving as .bmp because they load really slow and I think some people can't see them.

[cheeky]

JPEG (.jpg) or PNG (.png) is probably the best bet.