Yeh, it's only with progressive jpgs it does this I believe. Progressive jpgs save alternate lines first, then the ones in between, so you can get the idea of what the whole picture's going to look like while having downloaded only a half of it, although it means the picture has low detail until it's finished downloading. I'm not sure, but I think IE waits till it's finished downloading a progressive jpg then displays it, while Firefox displays it as it's coming in.