Cameras don’t cryptographically sign the images they take. Even if that was added, there are billions of cameras in use that don’t support signing the images. Also, any sort of editing, resizing, or reencoding would make that signature invalid. Almost no one is going to post pictures to the web without any sort of editing. Embedding 10+ MB images in a web page is not practical.
The cylindrical lithium cells can go off like a rocket, but the pouch cells in a phone just burn and are very difficult to extinguish.
Apart from the Galaxy Note 7 which had a design flaw in the battery, it’s incredibly rare for a phone battery to catch fire without physical damage though.