rogerd - 12:10 am on Jan 4, 2010 (gmt 0)

One advantage of uploaded avatars is that you have more control over what displays. If the avatar is hosted remotely in a place controlled by the user, it's easy to change an acceptable image to something problematic.

A more complex alternative would be to allow uploading to another location you control and linking from the forum to those images. This keeps your prime location secure and also lets you offload some bandwidth if that's an issue.