From the Model Mayhem FAQ:

What payment methods do you accept?
We accept all major credit/debit cards: Visa, MasterCard, American Express, and Discover. We also accept PayPal!

Please note that while our billing system might be able to accept some pre-paid credit cards, not all pre-paid or gift credit cards will allow charges for recurring billing cycles. If you are using a pre-paid or gift card please check with the card issuer to see if they will process the charges from Model Mayhem before you enter your card.