I just changed my avatar to a custom picture, I only just did this as I didn't even realise there was an option that I could do this, as the page isn't very clear!http://community.giffgaff.com/t5/user/myprofilepage/tab/user-icons%3Aexternal
Click above to change your avatar! You could use a picture from the internet, although it will be resized to match the size of the other giffgaff avatars.
Hope some people find this useful, I didn't even realise I could do this, haha! ;D