From what I understand reading this thread, if instanceA goes down, any images hosted there are lost, while the comments will still exist because they’re federated. You’re only shifting the responsibility of hosting the image from a site like imgur to the home instance of the poster.

I guess it comes down to if you’re concerned about how long your home instance is going to be around for, use an external host, or see if/when account migrations are added if images move too (although they would also have to fix the src for wherever the image is now being hosted)