It's possible to set the minimal avatar size larger than the maximum..
Which gives a somewhat weird error when trying to add an avatar

Warning

The submitted avatar is 176 pixels wide and 144 pixels high. Avatars must be at least 150 pixels wide and 150 pixels high, but no larger than 90 pixels wide and 90 pixels high.

Perhaps a small check might be in order to prevent unaware and still sleepy admins for setting stuff like this...