How do you change you picture to a custom picture?

You cannot.

You have may have been able to once, but that no longer exists as an option.

Same as changing to a bunch of the X-Wing pictures - they were previously available in the menu, but has been remove.d.. People who had them can keep them, but no-one new can add to them.

Ahh dang that sucks, ohh well had a cool picture too