You might be right, I just noticed that it started happening at the time CloudFlare was introduced. It was never so slow that you would read a post without even knowing there was an image missing before. Also, I'd never seen the 'Invalid Proxy' message displayed when there is a link to a valid image. Whatever it is the post hasn't been changed and the images haven't been moved so it is something about the forum that is preventing them being displayed.
Sometimes, when loading a embedded image, this picture is displayed.

If you refresh the page several times, the normal image is loaded.