I assume, that Stack Exchange have some clean-up subsystem, which deletes all "abandoned" images (stored at i.sstatic.net), that is, all of them, that were initially uploaded, but no post is actually having link to them. Am I right?
If such subsystem is implemented and is doing its job on regular basis, then what about following scenario:
Start a new question or answer, anywhere.
Hit button and upload an image.
Copy generated image's URL to clipboard.
Close tab and drop editing post.
Go to question on answer, you actually planned to reply.
Post a comment to it and paste copied image's URL from clipboard to comment's body.
Will uploaded image, with URL pasted only to comment, be kept permanent? Does clean-up subsystem (if exitst) take care of checking for image's URLs in comments as well?