Ex wife sends pictures

My husband's ex-wife sends pictures of their son with captions like these "look how cute our son is" "we did a good job" "our son was great today" mind you we have full custody of him and she only gets him every other weekend. My frustration comes with him responding to her text.  Like they are a happy little family. The horrible things that she has planted in my five-year-old stepson's mind and the things she has done to my bio son makes me hate her worse than a "normal" ex-wife. Am I being petty for being frustrated with this? I don't want her texting him!