Why? Because the timestamp should commit to all the content, including the image itself.
For example, here's a lovely photo of São Paulo:
At any moment it could be undetectably replaced with a photo of Craig Wright! With hashes, we can detect such treachery. Without, we must trust @k00b
Good point! Linking to an external source is a gap in integrity
So images are always hosted on stacker.news servers? It's not like the OP can change the image later or if OP gets hacked, attacker can change the image. Only @koob or someone who hacks koob can change the image?