The use of background-images?

Hi guys!

There are a few background images I'd love to use on my forum but despite my best efforts, I was unsuccessful in adding them in any meaningful manner. Would there be any helpful pointers and hints on using background images? For examples, which format of the forum-software actually does support them and once that is figured out, how does one actually go about adding them successfully?

Every time I attempted to modify the "suggested" background image, it would occasionally display it and then promptly forget about it again... also, are there ways to lock the size of the image despite the size of the viewers screen? Etc, any tips and hints would be brilliantly useful!

Thank you already in advance...