HungryEye wrote:
Might be your colorspace.
How are you converting to jpg?

You should be using the "Save for Web" tool, which will convert an image to sRGB, which is best suited to viewing on the net.

+1

Most browsers don’t understand color manaqement. Your images need to be converted to sRGB before uploading to the web.

See the Color on the Web section of the Color Calibration and Management Reference thread for details

What can I do to make my images look good on ModelMayhem?

Start with a good original. 
Make a copy and resize it to no more than 800 pixels wide; anything larger will be resized by MM, and quality will be lost. (400x600 is a decent size, or 500x750.) If using Photoshop, Bicubic or Bicubic Sharper should be used for the interpolation method.
Convert the image to sRGB. (Photoshop: Edit > Convert to Profile, make sure Black Point Compensation is enabled)
Sharpen using your favorite method. (Unsharp Mask at 100, .4 pixel, Threshold 2 is a decent starting point for many images at this size if downsized using Bicubic.)
Save As… using JPEG filetype, and compression of 7-9 so that it’s under 500K in file size.
Upload to MM

That’s NOT the only way, but it’s one good sequence. If you use Photoshop, creating an Action to record these steps will save time, and help avoid missing steps.

For more information about the color issues, see the Color Calibration and Management Reference thread.

P.S. The DPI setting makes no difference. 72, 300, 1, 1000—it doesn’t matter for the web.