Did you know fantastic help is an anagram of Planet Catfish? This forum is for those of you with pictures of your catfish who are looking for help identifying them. There are many here to help and a firm ID is the first step towards keeping your catfish in the best conditions.
Generally, for sexing, you want one fish in each picture, against a clear, high contrast background [such as a dark fish on a white background], straight from above. That generally gets the best image.
Taking one fish at a time out of the tank into a white plastic icecream tub, or similar, would be the best way. If you've got more than two fish, it's probably best to take them out of the tank into a holding container (bag/bucket) and then take one at a time out of the holding container into the photo contiainer, and then back into the original tank - that way you don't have to figure out which ones you've got photos of and which you haven't.