That thread explains how to size your pictures for display here - if you don't use Photoshop, you can always ask in one of the forums and someone who uses the same software will be more than willing to help you out.
As for the photo, I suspect you're trying to play up on what appears to be "Alien Workshop" - the greenish tint to the light accentuates that well. Good focus and framing, though the bluish background appearing on the lower left is a bit distracting.
Had to study this a bit to understand what I was looking at - a ball cap.
With the maximun image size of 720 pixels available on the longest side, this one at 500 pixels is probably hurting you score-wise than anything.
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