From the Model Mayhem FAQ:
I'm being told that I am over my photos limit?
This is a result of you once being a VIP or Premium, getting access to the increased photo limit, and then having your VIP or Premium membership status be canceled and so now your limit is back to its original limit. In order to add new photos to your portfolio, you can now do either of the following: 1) Sign up again by going through the registration again. 2) Delete photos in your profile until you get below the limit of normal membership.