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From the Model Mayhem FAQ:

How can I add images or fancy text to my profile?
Images and fancy text are available only to Legacy members, and members who have subscribed to Model Mayhem. (You can subscribe here: www.modelmayhem.com/upgrade)

The coding used for this text is called BB Code. We accept most types of BB Code, though some tags (such as for videos or youtube) do not work. You can learn more about BB Code here: https://en.wikipedia.org/wiki/BBCode