I just checked in my solution. I have made an image expire and try to add it to WYSIWYG editor. Episerver Admin UI did not show any error and I still can drag & drop the image in WYSIWYG editor however when I restart iis & use Guest mode (or another browser) the image does not show. So Image expire did work (i have not used it in any project.)

Hi Sam, I believe you are facing the issue with the image being cached to your browser. So when the image expires the link to it wont get removed from the TinyMCE produced HTML and when you browse to the content again after the image has been expired the browser has that image cached and won't request it from the server (test it with your browser, use developer tools to not to use browser cache or use another browser that you have not yet used to browse to the content). As a side note this same would happen if you first have some content (image, a file download) publicly available and browse the content and then change the access rights - browser has already this file cached and it would look like the access restriction doesn't apply.