Change Background of Pictures Hub on Windows Phone 7
On Start screen tap “Pictures”
Then Tap and Hold on blank space on background of Pictures hub, a menu with two options will display, so choose that according to your requirement.
1- Change background :
Tap on Change background option, and browse for a picture from your saved pictures.
Then simply tap on your favorite picture and confirm the selection by tapping on tick mark, immediately the Pictures hub background will get change.
2- Change it for me :
This option will randomly select the pictures for Pictures hub from all of your saved pictures on mobile.