Beforehand, it was ok as long as there was a character from the original trio in the artwork.
However the rules have been changed since then. They are written in a way that the characters from the main trio are always the main focus. Other characters are welcome to be in the art, but the trio MUST be there too.
The art you speak of was submitted before the rule change, so it's grandfathered in
(Although obviously this club would accept pokeshipping, gymshipping and bouldershipping; it's not a shipping focused club. The other ships are welcome as long as the other members of the main trio are present as well)