It's been my experience that saving a JPG right out of the maya software renderer never yeilds a high quality image. So far I haven't found a setting that adjusts JPG compression. Usually it's pretty high and artifacty. Best suggestion is to render to a different format, preferably lossless (TIF, TGA) and then compress in photoshop or some other program that allows you to set compression.

It also looks as if you have an ambient light turned on. You might want to go with a 3 point rig instead to get the lighting a little better.

Hope this helps.
-Dave