In this Maths Meta SE thread, I noticed that the image linked in my answer was blurry, even though it was being scaled down to a lower resolution. For comparison, check out the images below:
The left image is the original image, and the right image is the scaled image, where both images are scaled down to medium (320x320 pixel resolution of thumbnail).
I've also tried using HTML to resize the images, but the Imgur image didn't show up that time, so I can't check how that affects it.
Why is this happening to my images? Is the compression happening on Imgur's servers or the SE servers?