I found that it worked just fine for me, at least for the one post I tested it on. I edited a previous post, entered a brief description in the box, then when I clicked on "Insert Image" I just went to the gallery, selected the image, then clicked on "Insert into Post". I did not re-upload the image. Updated, the post, tried it out, and everything worked just fine. Looking forward to seeing how it works over the next few weeks and months.
The only thing I would change is the ability to specify where at the end of the post the Pin It button goes, and add a counter. But since I don't want to use a floating counter, my choices are pretty limited right now until this one is updated or the Sexy Bookmark bar adds Pinterest to their options.