Seems pretty simple to me....the mod/coord shouldn't have to change the entry to fit the rules for the contestant.
If the image does not conform (ie wrong size)...send it back to the entrant.

I know this is just a fun contest...but still, if folks ever want to enter a real competition....they had better not expect organizers to make their image conform to the guidelines....

In youth, we learn....with age, we understand.