Setting an avatar for a main site that is not the admin's ugly mug

This is on a BP multisite installation, obviously everyone who signs up has the opportunity to choose an avatar for their BP membership, but not, as I understand it, the choice to have their blog use a different one. This won't be a problem for most of the members but it is for me. My gravatar is in place by default and was set before all the other installations were done. But it is not appropriate for the main site of a network to have my picture representing the main site.
It seems though that there is no way to change this, my understanding of the BP avatar function is that it is open to everyone except the super admin.

Does anyone have any bright ideas?