hmm, that's called a banner.
you need software like.... adobephotoshop, or paintshop pro..
when you make a background, you drag the picture that you want into the background.
after that you save it as png, and upload it to
imageshack.
then when you get the properties of the uploaded picture.
you go to your control panel, and go to edit signature then do the following...
[code:1][IMG]whatevr your properties of the uploaded banner is[/IMG][/code:1]
then there you have it, you're done!