They can change the content, you’ll just know they did it. Much like if my watch gets stolen I’ll know, but I still don’t have a watch anymore.
Sure you can back up an image, but the backup is worth the same as a copy of the NFT: zip. You now own a pointer on the blockchain to nothing and a jpeg on your disk. I’ve got a lot of that going on already with zero expenditure.
Similarly, you can prove to others that the version on your disk the version pointed to by the blockchain by having people check the hash.