Yeah. CAS poses aren't for the purpose of making your sims pose pretty in the gallery - there are different poses altogether for that that instead alter the pose in your sims family portrait. Instead, you should use CAS poses to take pictures in CAS - press c to take a screenshot without the UI (making sure capture UI is unticked on your settings.)
Anywho, gallery poses, you can find a few here:
https://j-e-n-n-e-h.tumblr.com/Basically, when you go to play with your family and you see that picture of them all posed together, press the camera button to randomize the family portrait - it will put them in different poses. Each sim will go into a random pose, so you may have to try a few times to get it just right if you want certain poses or poses that are corresponding. Also, the more copies of the same gallery pose you have in your mods folder, the more likely a sim will be to do that pose. I'd advise making a folder on your desktop specifically for gallery poses, and then when you want certain gallery poses for a family you want to upload to the gallery, take out only those poses from your gallery pose folder and place them in your mods folder so that you don't have to keep randomizing the family portrait to get the right poses (more gallery poses in your mods folder = more randomizing for the right pose.)
If your family wrongly gets marked as having mods bc of the pose, turn your mods off and the family portrait should stay the same and you should be able to upload the family without it saying they are modded when they are not.