You can choose "share" from the list, then "photo". After that, it's best to have a Flickr account, or at least an account on an ISP.

You fill in the link box, add what text you want, edit it further after clicking Enter, and then Say it.

Thank you Miss Harper G

Whoops; I just realized. Were you talking about the _timeline's_ background, the "main screen"?

If that's what you meant, then you have to play with the CSS of the page. They show you how to do it under your Profile Edit.