Sending photos to him Basic Training

Hey so my boyfriend is in basic training and I am going to send appropriate cute photos of us and family photos. But I was wondering is it fine if I wrote on the back of the photos with black pen? It’s just like a sentence or two. And is there a limit of the amount of photos we should send? I was gonna send 4 photos. I don’t want him getting in trouble so I wanted to ask beforehand. I think it’s fine but what do you guys think? And don’t worry I did not decorate the envelope. I just drew cute things on the side of my letter. Also he is in the army for infantry airborne.