Hmm… that’s strange, I’ve never noticed anything like that when posting. It almost sounds like some kind of internet connectivity issue because it reminds me of how photos (especially in the days of much lower bandwidth speeds!) would load progressively like that and sometimes time out.
I’m curious what site/app you’re normally posting through? I’ve gotta admit I’m not sure if they’re all sharing a unified image server at this point or each host their own.