Go into "personalize my homepage" under privacy & settings. click on "advance" (or something like that. on the top left where it has all the colour options for your tabs & stuff the top option lets you change your background pic. You use that to upload a pic from your computer. I just did this last night...my memory is fuzzy on the exact wording cuz it was late but I hope this helps.
at 11:24 AM on Jan. 5, 2009