I can send picture messages to anyone. Except for my husband. We both have S21's and it used to work just fine to attach a photo to our texts and send them. One day, just quit. I use the messages app. I cleared the cache and rebooted my phone. He can get photo texts from other people.
I can't imagine not being able to send pictures of the kids to my wife so let's get this resolved! Have you tried a different messaging app or tried removing the prior conversation completely and resending a new message if possible?