I think that you already know the answer. A medium sized Carp or Catfish wouldn't even be able to turn around in a keepnet. If you want to take a photo, keep it in the landing net for the shortest time possible to set up the camera and then put it back. To keep several fish in a keepnet of that size for a grouped shot at the end of a session would be highly frowned upon in the UK. Sorry if that sounds a bit harsh but I think that we take our responsibilities of fish care very seriously and it ensures that our stock survives and is able to flourish, despite some fish getting caught several times a season.
There are some knowledgeable and helpful anglers on this Site so feel free to tap into it and expand your understanding of the sport and the standards that we aspire to.
Good luck.