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You could allow them to create an avatar of their face/head and upload it as an avatar.
Or, there is a nice hack you can use to allow photos. There are two versions, one for XOOPS 2.0.13.2 and for 2.2. I am using the one for 2.0.13.2 on a site of mine which is running XOOPS 2.0.15. I just made sure I did each file which needed modification individually instead of overwriting. Just to be as safe as I could with being sure to keep any changes/bugfixes 2.0.15 brought to us.
Do a search for Photo + Birthday Hack and you will find some threads. Click on the one which is for your version of XOOPS and it will take you to the page where you can download the hack.
Always Backup Your Site and Database before making these kind of changes!
Best of Luck!
Quest