Whenever I upload a set of large images, it really slows the browser down, because the image previews embed the entire image in the page. The latest set I did with Firefox caused my entire computer to stop responding until Firefox eventually stopped the JS from running.

We could disable previews for images over a certain size, but that might not work for releases with lots of images and it means we lose the benefits of having thumbnails in the first place. It would be better if we could find a way to show actual thumbnails. The answer on http://stackoverflow.com/questions/13905578/reading-large-images-as-thumbnails-locally-via-html5-filereader contains some links which might be useful for that.