I've noticed in my group's photo folder, the images that form part of people's signatures are displayed enlarged in the list view, and extremely enlarged in single view. I'm confident that the original images were not this big.
When I click the Download button, it opens the image in its original size in the browser (which, I suppose, would be confusing to some newer computer users who would have expected to see a download dialog pop up, but that's another issue...)
I also uploaded a test image which is crisp and clear when viewed at the actual size (which is smaller than the browser screen), but when I view it on the Groups.io web site, it looks blurry because the system slightly enlarges it.
So: please don't enlarge images when displaying it in either the [thumbmail] list view or in the individual view.
(I'm using Opera with Blink rendering, by the way)