Why not write a server side script that would allow users to input ad-hoc an external image link, it would then be tested for various properties (i.e. the size of the file, whether it is a valid image file, whether it is a dynamically generated image, whether it obeys the dimension restrictions) and if the image passed the criteria, permission would be given to the post parser to display the embedded image. If not, then the image would not be embedded.
When the images are external, the image itself can always be replaced later. So just because it passes today or tomorrow, in 3 days, I could keep the same image url, but the image itself is different. The only solution is local images or no images. Otherwise, there truly is no guarantee.