Basically there has not been any strict rule about pic updates so far. So, we really can't force any member NOT to update others' photos. We generally request members to wait for a while to let the blogger update his own pic. So, our norm is very simple. If the blogger becomes first, he gets priority and other member's entry gets voided. If by chance, the member updates it before the blogger, then we accept both of them and manually remove of the duplicate pics later.
Thanks.