Here's a "tack on" to Ticky's instructions. I tend to work the URL's of pics the same way Ticky does it. However, when you pull the picture up to full size, if you look down at the bottom of the page the picture is on, there are two internet addresses...one is the straight internet address and you can just highlight that one and copy it. The other one is The same address bracketed with "IMG" tags (this one is cool to use if you want to insert a picture into a forum message.
One other thing. The addresses are a bit long and you can't see the entire address in that bar underneath the picture. However were you to scroll all the way to the right, you'll notice that there is a 3 digit number and that number is all that changes. At least I've only got 3-digit numbers. And I've only seen that part of the address change.
So, you can upload the images you think you might want to use for backgrounds, past the first one in all of the bars on the "Customize Profile" page and then come back and just change the last number in the address to get the changes made without having to cut and paste every address.
Rhonda touched on something very elementary in web design. She is FAR better at that than I am, and that's just a TRUTH! But, especially if you are working on a background that contains text, you're going to want to stay with a pretty bland design otherwise, viewers will get a headache trying to dig out the text from the background.
One other thing to keep in mind when selecting the main color of your background is that you have no control over the color of some of the text that is displayed. It is RED and forever will be RED unless Soda finds an option somewhere to give us to change it. So, if you select a background that contains a lot of red in it, that text is going to disappear on YOUR site.
I agree with Mike that one must be careful about one's backgrounds. I see it so many times - people find a background that's really cool or pretty, and they use it despite the fact that it makes the text practically illegible. I teach lessons on web design and it's always one of the most common errors. Heck, when I made my first site back in 1999, I did the same thing! Animated sparkling stars as the background... yikes!
If you have Photoshop or a similar program, I suggest either lightening, darkening, or tinting any background images that have a lot of different colors in them to increase text legibility. I would also recommend avoiding image-heavy backgrounds unless you have darkened or lightened them considerably so that they become subtle background rather than dominant elements. Of course, when there's no text to worry about, you can go to town!