That is quite limiting in my opinion because an image can distort and subconsciously limit ideas.
I feel we could (also) have a competition whereby we have no images nor descriptions of images at all and concentrate solely on what the card does.
I think having competitions with limitations is much cooler than having:
Generic Design Any Card Competition #1
Generic Design Any Card Competition #2
Generic Design Any Card Competition #3
Generic Design Any Card Competition #4
Generic Design Any Card Competition #5
Generic Design Any Card Competition #6
Generic Design Any Card Competition #7
etc.
The idea is to get images that can be interpret in many ways. I think trying to find a card to match an image is a very cool exercise in creativity. And like I said before, the best part is that there's a bigger chance for the cards to becoming a reality because they all-included package.
That second part you say was my other option but I think it would have been a bit boring.