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From the Model Mayhem FAQ:

What is VIP and Premium and why do I hear so much about it?
Model Mayhem accounts are free to use, but we also offer an upgrade/subscription service that gives a boost to the basic features available on the site. There are different levels of upgrades at different price points: Premium will get you a substantial bit more than a basic membership but VIP is the best you can get.

You can see all of the features and options between the various subscription levels here:
www.modelmayhem.com/upgrade

Here’s a small sampling of benefits:
  • Increased number of photos
  • Increased number of daily private messages
  • Reduced time between front-page announcements
  • Increased number of casting calls and notices announcements