It appears that uploading large images (over ~4MB) will fail to upload and all you can see in the console is a 500 (Internal Server Error).
GET https://meta.stackexchange.com/error?aspxerrorpath=/upload/image 500 (Internal Server Error)
This should instead display an appropriate error message instead of just hang there. Perhaps warn about size limits of the images too.
I'm using Firefox and Chrome, latest