If you're talking about the Spaceship Earth tiles, just upload an image via your Albums then copy the "Picture URL" to your Customize Profile page.
Under "Main" (the first section) look for something that says "Background Image" once you find that paste the Picture URL into the empty spot below it.
If you want it to repeat through the entire background select "Both" from the "Repeat" drop down menu to the right of the background spot.
In doing this I would try to find a repeatable pattern (or close to it) so it fits smoothly across your page.
You then can continue to customize colors and images for other parts of your profile. Clicking the "Preview" button should show you what it currently looks like with labels. Those labels will help you on your customization process. Just match the labels on the preview button with the labels on what you're trying to customize.
If you have any other questions on the Customizable Profile let me know. I really like vBulletin and have seen this feature many times on other websites.
If you're wondering "Why is everything purple?" on this page it's because purple is the only color you can really see my name in at the top of the page. That Spaceship Earth background is hard to work with.