This is something I'm patching up for 7.2. So instead of inserting a blank which some installs of MySQL won't accept, it will insert a 0. This is a quick fix by editing admin/changeuser.php. Line 171 of that file has these 2 lines:
$imagewidth = "";
$imageheight = "";
Change those, to this:
$imagewidth = "0";
$imageheight = "0";