I have only one question: did you modify anything in register.php or includes/email.php ?
If you did, then 99% chance that that is the case. If not, then, as TheOracle suggested, check your email server logs, and check your spam box, perhaps the messages being filtered out as spam...
To make sure it works as supposed to, I just checked the fresh 4images code. The code that sends out emails to admin and to new members are identical and it works just fine. On my test gallery.
So, it seems the problem is on your side.