This new photo uploader is terrible. The one thing it does right is adding drag-and-drop; aside from that, there is no excuse for:
Blocking the browser trying to generate an “instant preview” data URI, especially for large images.
Showing no indication that an image is currently uploading, which makes it nearly impossible to figure out if a post succeeded.
Long waits if you click “Create post” while a photo is still being processed, with only a spinner in the bottom-right corner.
Absolutely useless error messages, which are admittedly a carryover from the previous photo posting page.
At least you fixed Markdown previews.