I think you'll see that each flight is different, and each cadre different w/in the flight - some will allow the picture contained in the letter & some won't. But, your child will be happy with any mail they get -- so mail something every day - sometimes twice a day. DS knew I was mailing letters everyday so he knew that when they got to go to mail he would have something.
But, they won't get to go to the mail room everyday, and might not get there for a number of days at the beginning. They have to march EVERYWHERE, and it's sometimes not convenient w/ the schedule to march a flight to the mail room. Your child will so "no mail from you" but that could be because they didn't get there.
I would put a number next to my name on the return address -- #1 for first letter, #2 for next one, #3, etc. That way if he got more than one letter he'd know which one to open first.
I would type my letters and save them so I'd know what I asked or written and try not to repeat myself too much. Often it was general ramblings just letting him know what was going on with the family but definitely giving words of encouragement to him.
I'd often include Bible Verses at the end of the letter that he could tear off if he wanted to keep that verse with him.
DS was a big baseball fan -- so sometimes I'd cut and paste the ESPN recap of his team's game in the letter. They couldn't keep newspaper clippings but the cadre didn't know I was sending game recaps in the letter. They would be a couple days old by the time he read it but he wouldn't care as he had no idea what was happening otherwise. DS is 2013 - his BCT Michael Jackson died and the cadre told them that one night and his next letter said "did Michael Jackson really die?".
A few times at the beginning I included a note full of questions that he could fill in the blanks for answers or circle the answer and then just mail that back. (how many roommates do you have, what is their name, where are they from, where do you stand in the flight / squadron formation, etc)
Most of all keep encouraging them - to get from one day to the next, one hour to the next, etc.
Good luck