most of us mortals try to make order of chaos, we even see elephants and faces in clouds, it's charming. The Chuckster is insane in an evil, paranoid kind of incarnate way. This picture of Billy is not a current school picture, I am sure that all the latest school pictures of kids are passed around and perhaps sold in prison. The backgrounds are way znazzier now.
Knowing of his less than stellar past, I am quite sure Manson has a few crimes under his belt we are unaware of. It is quite possible that he believes this is one of his early victims come back to haunt him. Even all this could just be in his head. Not saying he killed Billy, but may have victimized a young blond boy, possibly named Billy, possibly in a shed like the one in the picture, hell, he could believe he killed Billy even if he didn't. Most serial killers like to keep momentos and maybe the shed is like his souvienier, no matter how he got the picture of it, or one like it.
Looks like the letter might be an attempt to ward of one of his demons. The repeated twos, three times are just a reminder of whos' team he is on. He's just reminding Billy that he remembers and little Billy should not be playing on his turf because Chucky don't like it when people try to screw with him and even if he can't get out, his immortal homies are loose. Because , remember he would not be able to send an overtly threatening letter anyway.
We will never figure any of these creeps out really, but I have a bigger question.
Why the H E double hockey sticks are these maniacs allowed to receive or reply to letters from children anyway???