I Need to approve custom gravatar uploads. Any suggestions?

I need to allow my users to upload a custom profile image (or Gravatar), but this image should not become it's profile image right away. Instead, some admin should check it out first, and approve it.
I don't know exactly how to do this. Do you have any ideas?

A first approach could be an extra profile field for a second image, alert the admin by email when user upload a file, and if admin approves, then replace old image with new one.

Any plugins that can help me achieve this goal?