If my XH sent me anything and I said thank you to him, he would accuse me so fast of cheating my head would still be spinning.
I think that THIS in and of itself is unhealthy.
I am all for not speaking with ex's. If mine and I weren't trying to coparent, I wouldn't speak to mine. I love that H doesn't speak to his ex's, and I love that all but one of mine are out of my life completely.
However, the simple fact is this: someone sends you a picture, and if you say thank you to them, you are accused of cheating.
I find that a really unhealthy place to be in general. Cheating aside, if my H's ex contacted him for whatever reason and sent him something that he wanted (that wasn't offensive or disrespectful to his marriage...I think pictures of a child qualifies as that...it's not like she is sending dirty panties in the mail or naked photos of herself), and he messaged her to say a quick "thank you" and that was THE ONLY CONTACT they had, I don't think I would get angry.
I seem your H creating a situation in which being so much as civil to an ex is unsafe and unacceptable...and then violating his own boundaries. I don't think that is healthy, and I too would resent the double standard.I have been cheated on by 3 different men, and I have more DDays than anyone ever should. I am here, just trying to pickup the pieces.
At least the current man "only" cyber-cheated.
"Love means never having to say you're sorry."