There are several issues with embedded images in posts that link to outside hosts:
- These images are likely to expire at some point, which can render the post meaningless
- External images allow users to track visitors of the page the post appears on
- Images from non-HTTPS hosts will cause issues once SE switches to HTTPS
While the second item is what triggered my feature request, I think the first one is the bigger issue. And it is an issue that is far easier to fix early than later when the image is gone. This check should also not be applied retroactively, but only for new posts.
I did a quick check to get an idea on how many images are embedded from other sources, and on Arqade 119 images are on non-imgur hosts compared to 7258 images in total. I chose Arqade as it is a reasonably image-heavy site and one of the larger sites in the network without being too large for the query to time out.
Images from external sources are a small minority, but not so small that one could ignore this entirely.
To avoid annoying users too much when prohibiting them from using images from external sources, the option to upload the image to SE imgur could be added to the notification when such an image is blocked. This dialog would of course have to mention the licence implications of uploading the image.