So, taking down another long-standing complaint. Two things were happening:
I've now got a sort of image proxy going on idkfa. It will rewrite the img tag in your post, and rewrite it so that it goes through a slapped together proxying service. The image's width and height with be recorded, and will be used to hard-code the width and height of images prior to loading, stopping the browser from reflowing each time a new image would load.