From the Model Mayhem FAQ:

How do I resize images for Model Mayhem?
Unfortunately Model Mayhem does not offer any resizing tools., but images uploaded to any account on Model Mayhem must be a specific size to be successfully uploaded.

Minimum Size: Photos must be greater than 200px wide and 300px high and 7kb in file size.

Maximum Size: 4 million total square pixels (e.g., 4000px x 1000px), or 1MB in file size.

The best way to resize photos is with a an image editing software on your computer, such as Paint, GIMP, Photoshop, etc.

If you don't have image editing software on your computer, you can use one of these online utilities. There are more online if you search, but these do give you some options.

We recommend that you resize your photos to be 800 pixels wide or less.