I believe is what is happening is that the client has 2 versions of the image, One for message showing, and one once you click on it (full res) and the media server is having a stroke cause there is so much going through.
So i believe that the image server is having a stroke (yes i said this twice) cause it's gotta download all the files and then distribute them to everyone. Which is probably clogging the upload.
Thread
Comment by @SkippTekk