I would love to know if I am being oversensitive here!
My MIL lives in a different country, so we occasionally send each other messages over WhatsApp. Since giving birth to her grandson, I send her pictures now and again, which she requested, but rarely responds or acknowledges the images with a 'like' heart, etc. She's pretty tech savvy, and I can see she has seen the photos or messages, so it's definitely intentional. If my husband sends photos, she is very responsive and always on her phone when we are together.
It's a bit like this with personal messages too, it's hard to have a dialog because she just chooses not to respond, and then will message a week or two later on something entirely different. For example, I ordered something to send to their house with her approval, and instead of letting me know it arrived, when I sent her a message to say it was out for delivery. She messaged my husband about it!
I don't care that much about the personal messages, even though I'd love to have a more consistent chat going on. But I find it really odd not to react to photos or make a comment.