Hi enrewen,
So, you have successfully uploaded some JPG files but not all. And size-wise, you unsuccessfully uploaded a 677KB sized JPG file but successfully uploaded 1.x and 2.x MB sized JPG files. All with the same Win Chrome browser, right? And the error simply said "HTTP error" and the filename, right?
Have you tried a) deleting your browser's cache (see https://support.google.com/chrome/answer/95582?hl=en) as well as a different browser to see if you get the same results? Moreover, even with Win Chrome, if you were to try to upload the five files again would you get the exact same results? Every time you try?
Hmm, and speaking of the five files, what are their exact names and please note which ones pass and fail the upload test. Oh, and please list the exact steps you take to upload the files. That may also help shed some light on the subject. That is, as Sky Marshal Tehat Meru on Starship Troopers (1997) said, "To fight the bug, we must understand the bug."
Yes, I am a NERD. ;-)
Good luck and know that I am leaving for the night but will check in tomorrow to see how you're doing. Hang in there and also know, most likely, this issue shall pass and you'll be flying high soon.