Hey Craig,
What you want to do is write a plugin that will add the picture to the vcard download stuff. We have some great documentation on writing plugins in general, some of which I've included here:
Building Plugins
SBS 3.0 Documentation
What you're probably gonna have to do is change the /template/global/view-profile-vcard.ftl file, which you can do through the admin console > system > settings > themes. You can then add the line for a photo using the vcard photo syntax. I found an example of this syntax for you, it looks like so:
PHOTO;VALUE=URL;TYPE=GIF:http://www.abc.com/dir_photos/my_photo.gif
Just get the url for the user's avatar or photo, and that should be it. Let me know if you have any other questions.
Thanks,
Kevin