I'm going to get defensive when someone tells me to remove pictures of a car crash on the group chat.
Why would you get defensive about being asked to delete something so horrible?
Because the pictures are not hurting anyone.
If it made people uncomfortable in the group you're working with, then you should get rid of them.  What's important isn't what you thought.  It's how the audience responds to what you put out there that matters!
I just don't know why people are not understanding and they are so offended by my action.
Because car accidents are upsetting to look at and it's unnecessary for you to show that kind of unrelated image on a group chat.