the question: Once you reset the "large" size in Media settings, how can you retroactively convert all the old files to this new size WITHOUT having to re-upload everything?
my specific need
I'm using Wootheme's excellent Object theme for a client's site.
The theme takes the users "large" size images and displays them in a sliding gallery.
You can see this here.
Unfortunately, the theme wasn't built with tons of large images in mind, and it loads ALL of the images and THEN displays them. I'm looking for a solution that will load them one by one, but in the mean time the solution is to size all the images to the dimensions being displayed, instead of WP taking the "large" size (1024px wide) and then scaling it down to the display size (580px side).
So I've modified the "large" size to be 580px wide, so future uploads will be optimized, but what do I do about all the old images? I don't want to have to upload everything again, as there are A LOT of photos.