All right, we've fixed the issue.
What should happen:
* When you upload an image and add it to a post, that image is moved from your shelf to the post. It will no longer reside in your shelf.
* If you want to use an image again, you need to upload it again.
On some images on the site, you get a contextual menu with the option to 'copy to my images'. This is a way of reusing images that are on the site without having to download and reupload the image. We are planning on rolling out this image menu to all images eventually, so reusing someone else's answers image is as easy as clicking 'Copy to my images.'
What was happening:
* When you put an image on a post, it did not remove the reference to the image from your image shelf.
* Then, if you ever decided to clear your shelf by clicking 'Delete All Images,' it removed them from all the posts the image was on. This is '''a very bad thing.''' Removing images from your shelf should never cause data loss.
I'm terribly sorry about this. We take revision history on posts very seriously, and there should never be any way to lose any data from a post.