I must admit that the correct way would be to talk to people in advance and ask for the required resolution, as you suggested. Unfortunately, that did not fit my last minute conditions I had during the last years. Maybe this year I’ll start a bit earlier. But I must admit that personal correspondence to more than a dozen people worries me a bit as a non-native speaker. Answering or posting in the public part of the forum is still something else than personal messages.
But for me there is a reasons that anyway speaks for high resolution images in the threads: The things we post here are under a cc license anyway (I guess unless noticed otherwise), and e.g. in the PlayRaw threads, there is even a raw image available cc licensed. Giving away cc licensed images means that others should be able to use the picture given they follow the licenses requirements. What sense does it make then, to post a low resolution image? Does it say “yes, I give this away cc licensed, but I give you only the resolution to print on a stamp, not the resolution to print big and hang it on the wall in your home”?
IMO the traffic problem has to be solved technically, such that only when download is clicked (or the web view is clicked to enlarge the image), the full resolution should be delivered by the server. If this is not yet possible, an upstream bug has to be filed against discourse. If I do not want to allow people to hang my cc licensed images on their walls, I am still able to upload lower resolution, but this should IMO not be the forum’s general policy.