So I dragged my sorry Pfizer second-shot ass out of bed and did some experimenting and found that a 1280x960 72dpi JPG at High quality creates a 229k file and uploads without a hiccup as an attachment to my post. I tried the same export but with the JPG quality set to Maximum and it choked on the 1MB file. So the limit is somewhere between 230k and 1Mb.
For real geeks, the photo is from my iPhone 7+ on full auto, then opened on my Mac in the stock Photos app, then exported with the setting described above. All photos were rendered in plain ol' RGB.