You still have to load it to see it, so I'm not sure what good it'd do to go through all of that. It doesn't slow general browsing down, since the thumbnail is displayed on the front page and in looking through graphics. It's really more of an issue of images that big not being uploaded as true-color bitmaps. JPEGs and PNGs are just as effective, and aren't a megabyte in size.
ARE YOU OUT OF YOUR MIND?!?
Are you aware that dial-up users frequent this site?
I mean, it looks nice, but really.
Is this a big problem?
Loading them based on file-size would be a lot trickier though - something that big should be postponed till a Drupal 6 port.
More of an annoyance.
Can you convert it?