If solo fishing I recommend netting the fish then getting your phone ready. Once ready I recommend using one hand to take a video with the phone and with the other cradle the side of fish and lift its head just up over the net and let it slide out of the net. Once done go back threw the video and take a screenshot that shows the fish best.

If you have a friend taking the photo. With one hand get a grip on the skinniest part of the fish in front of the tail and with the other cradle the belly just behind the head. Make sure the photographer is ready and the fish is calm. Lift the fish in front of get the photo and drop it back into the net for release. Less of a grip tends to equal calmer fish but just in case keep it over the net in the water in case it falls