In many environments there's already an applications that provides avatars.
The avatars may be available on URLs like this:
http://myserver.com/avatars/byuserid/

{userid}

http://myserver.com/avatars/byemail/

{useremail}

http://myserver.com/avatars/bymd5email/

{usermd5email}

If this plugin would allow to customize the "gravatar" URL it would allow to use these applications as avatar providers.
The default should be http://www.gravatar.com/avatar/{usermd5email}

of course.