Can I ask why you're nervous about her knowing you spend time in his classroom? I think I'm a little confused (and it's probably just because I'm not familiar with your back story

). Are you afraid that if she finds out she will try to have him removed from the school?
If it were us, we'd probably just let her see it and if she confronted us about it we would say something like, "Oh, it was sort of a last minute invitation to join the picture by the teacher. Occasionally I spend time with SPED students in (dss's) class and the day the pictures were done happened to be one of those days. We didn't mention it because we didn't think it would be a big deal. Sorry!"
Something like that is honest, to the point and also adds in that you're sometimes there to work with your own students, not dss.
Saying something about it beforehand might make it seem like a bigger deal than it really is.

It might also be taken as you being really quick to defend yourself and thus possibly be going out of your way to hide something (which you really aren't - I genuinely don't see why she would make such a big deal out of it if she knows you work at the school in the first place

).