Hi, I've just noticed this problem recently actually after switching from a fast Internet connection to a slow one (~0.4 Mbps at best, varied due to the nature of the mobile connection).
I believe because of the slow connection, the browser may be requesting a lower-quality version of the image, because when I open local files in the browser, the image quality is fine. (I see you have experienced the same thing.) I have noticed this quality reduction in PNG files occurs in bit-depth, and in JPEG images, quality reduction occurs in the compression, resulting in artifacts.
One thing you can try is opening the online image itself in a new tab, and pressing Shift+Refresh (I'm using Google Chrome.) For me, this forces the full quality version to be shown.
I'm unsure if there is any other workaround for this, as I'm not 100% sure what is causing this to happen, but I hope this sheds some light on the issue.