When I checked a few weeks ago , it appeared that Steemit wasn't actually compressing blog post images down. So to save people from downloading super heavy images, I exported at 1600px width (2x of the 800px max-width of blog posts).
Anyway, I just checked again. It looks like Steemit now processes down hosted images to a width of 800px. That's awesome—the devs around here work quick! Next time, I'll upload high res, knowing Steemit will compress down on their end.
Thank you for this useful information.