so... I'm not trying to be a dick, but using that HTML attribute doesn't make the page load any faster. It just tells the page to display it resized to a smaller resolution... it still has to download the whole image. If you look at the properties of each image loaded that way, they have the same file size as the full version. Perhaps I'm completely misunderstanding something here, but I think you need to have some kind of server-side script to save and resize the images in order to actually save bandwidth, which doesn't make any sense if the images are being stored on another website anyway.
To repeat myself, differently-sized/attached images are handled by different scripts, some of which are more efficient than others. Thumbs for attached images over 400x400 pixels are created and stored server-side, but they're converted to png format for display, so they're often larger (bytewise) that the unscaled original files. We know we need something better, but getting it will be difficult for a number of reasons.
So yeah, this forum software isn't very good at displaying images, and after we discovered that, the admins installed a couple of modifications to fix it, but those actually just made the thumbnailing even weirder.