Recently I asked this question on global Meta. The problem is when users upload huge photos to illustrate something they usually don't need very high resolution - for example, it might be a photo of a bicycle lock and it doesn't need to be 2000K by 1500K and weigh 700 kilobytes - it can be much smaller and will still look good. Downloading huge pictures is a problem for many users.
So you upload an image and you get a link like http://i.stack.imgur.com/Xulw9.jpg and when you insert
l you get http://i.stack.imgur.com/Xulw9l.jpg (notice
.jpg) and that automatically makes imgur.com serve a smaller version of the image that weighs much less.
I kindly ask all users who add pictures to use this feature unless of course you need that super high resolution to illustrate some super fine details.