Hello! I suspect that this is an issue with the forum software and the upload cache... It'll be sorted out as soon as possible, but that might not be until tomorrow as the ID people don't work on Sundays. Sorry for the inconvenience, and I would sugges that for the moment you use external links to place like Imgur until it's resolved.
I always get this issue when trying to upload image files that are too large (or gifs in general)
The embed option has never caused me any problems (other than maximum 10 images per post).
I upload all my images to gifyu.com, imgur works well too unless you want to upload a gif (they convert them to video files which means they can't be embedded).