I'm probably the last person in the world to hear about Repper (or at least the last person who might potentially be interested in it), but just in case I'm not, I highly recommend it, if you want to quickly and easily create a seamless tiled background image.
It's web-based. You don't have to download or install anything to use it. You don't even have to register. You just go to the web page, click "Start Tool Now", and play around until you get something you like. You can use their photos (one of which automatically pops up when you start the tool) or upload your own photo or other image. Move and/or resize the "sampler" square to repeat different parts of the picture. When you get a pattern you like, save it. (There's a brief "how to" video on the home page, if you need help.)
I found this tool when I decided to finally make my own background for my Twitter page. Not only is it simple and fast, but it's also quite a bit of fun.