k, i just made this in like 10 mins for my buddy Mythic. Hes like, the clan wheel man. My Main Focal was the driving spartan, so i just sorta blurred the one in the back.
First things first: Resizing to fit the sig size is generally bad, unless you do it in aspect ratios. not liek 50% height and 70% width, because then it looks scewed. Next, I would add some more background, maybe some grunge brushes or those C4D thingies that I've never used. Those could give a better feel the the signiture, instead of the black background. Hope this helps! Don't discourage, and keep going.
When you use a render don't distort it. I could barely tell that a spartan was on a mongoose. Try adding effects to fill in the space around the original render if you want to fill in the regular sig size area.
Here, when you're resizing something, ALWAYS hold control if you're using click-drag scale. I don't think you've understood what everybody's meant by not distorting the image.