From the Model Mayhem FAQ:

What payment methods do you accept?
We accept all major credit/debit cards for personal subscriptions: Visa, MasterCard, and American Express. 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, gift, or other electronic credit cards will allow charges for recurring billing cycles. If you are using a pre-paid, gift card, or e-card please check with the card issuer to see if they will process the charges from Model Mayhem before you enter your card, and have an alternate means of payment available.