I posted this in the other thread, but I can see it will quickly get lost. If you'd like to have a Gravatar unique the Xojo Forum you may find this tip helpful:
If you happen to use Gmail, you can create a unique address that goes to your address with a +. You can use this technique to customize your avatar for specific locations. It's also a great way to track down where spam came from.
For example, if your email address is
firstname.lastname@example.org then you can use
email@example.com in both Gravatar and the forum to get the forum software to display a unique Gravatar.