Nokia 6101 / 6102

I notice that when I try to use a photo that has been sent in an MMS message to me (T-Mobile) that when I use it as a Background it does not use the full screen. I have tried to vary the size of the picture in the email but it doesn't seem to make any difference. Does anyone know the answer? (Maybe this just does it on pictures received iva MMS messages?)
Thanks for any insight.