Please read!

A lot of websites with collections of wallpapers/screen backgrounds are NOT okay to use as sources for skin backgrounds. There's a key difference between using such a wallpaper as your computer's desktop wallpaper and using it for a skin that other people can use, and that difference is distribution. You, as the skin creator, are using someone else's work and distributing it to others without the original artist's knowledge or permission. The background websites you're crediting? Don't own the copyrights to the images and have a generic "Copyright belongs to their respective copyright holders," yada yada, text. Again, this is not okay. If you want to use an image that belongs to somebody else, seek out the original artist and ask their explicit permission. A screenshot of said permission will be required along with the preview of the skin.