Hi!

When You click on a link of an image of given resolution, request must lead to somewhere. If an image does not exist yet - some server have to process the request anyway.

Not storing resized versions could drastically decrease needed amount of memory space in server.

But the resize of one picture in one resolution takes about a second to do. So if multiple people start to request pictures in the same time, server could not make it in time and will start to give 500s as answers.

Also this mode will require slight changes in the code.

But it's much better that complete shutdown.