Now, I'm going to assume that you're on any form of CFW for 3.55, if not then I'm not sure how to do this.. but here we go!
1. Open up your FTP on PS3 and computer
2. Connect normally
3. Go to dev_hdd0-home-00000000X (This is the order of your accounts on your PS3, eg : the first is the first account in your PS3 in the XMB)
4. friendim-avatar-choose any friend you'd like to change-
5. Now go online and pick any picture you'd like.. save it as their PSN.png (For me it'd be Y2K_BUG_.png)
6.Drag and drop the picture into the folder and once you go back to your XMB it'll be change.. you're the only one who can see it, but it's still pretty funny.
so help with patch editing? please. ill give vbux