I've done a lot of them and the main criterion is to crop the chip from the background and have it of a sufficient resolution that it will show well. I usually submit them at 150-200 dpi and TCG staff seem to have no problem putting them in the right size. They dislike having to crop the image as that is time consuming and they have a lot to do.

Here is your 3-chip image cropped to one-chip and with the lighting optimized. You could submit it just like this if you like.

The page for submissions is at:

http://www.themogh.org/cg_submission2.php

Be sure you check the box that says you own or have permission to post the image or they will not accept it.