It's possible that I've misunderstood. And it's also important to note that I was looking into this for the purposes of creating my own, single user instance. I wasn't planning on posting to my own instance, just using it as a single logon where I could control what other instances I federated with.

Here it mentions not installing pict-rs and removing its configuration if you don't need image hosting. My interpretation at the time was that it would mean that no images would be hosted locally on my instance. But that was very early on before I understood more about federation, and now I realize that it may in fact also mean that any content coming from federated instances could have images broken, not that it would load the images from the remote instance. So now, I no longer think that this is a solution for not syncing images, but I'm not at all sure of that.