Lou -- problem fixed, it seems. Please re-try the upload yourself.
I found and removed a "feature" in the standard phpBB distribution, that was apparently "resizing" images to plupload quality 85%, even if the pixel count and file length were already within limits. This had a plethora of possible side effects, including altering image quality and color profile. I'm guessing it was the cause of the color profile oddness discussed HERE
I have now tested several image files and observed that the file uploaded and the file subsequently downloaded are binary-equal. It's a bit humbling to think that I must not have run that test before.
I have also added this mod to the tidy little procedure of only 46 steps, in my notes for doing the upgrade to phpBB 3. But almost certainly that count is too small, since I'm pretty sure I have failed to record some steps, and I rather doubt that the process is done yet.
Please let me know whatever else you notice that is odd.
Edit: resizing seems to be done on client side by plupload, versus server side by imagemagick. Or maybe it's done either place, depending on browser capabilities. This stuff is all too complicated for me to quickly understand in any detail.