1. Is this behavior expected?

Expected as in "You should do this" – obviously not. Expected as in "we know it's possible" – yes.

2. Does it not matter to Stack Exchange and Imgur?

It's pretty much impossible to prevent this 100%, at least up front.

However, hijacking our Imgur account poses a risk for the uploader: If they use their own regular Imgur account to upload images, they retain control over the image, and the one-hit-per-six-month threshold is trivially held for someone wanting to prevent deletion.

If, on the other hand, they use ours, that means they totally relinquish control of the image file, including the possibility to prevent deletion. If we decide to go delete an image that has never appeared in a post or chat message, they can hardly make an argument against us removing it. And they actually won't know until it's too late.

3. Is there already an active process keeping irrelevant i.stack.imgur.com images out of Imgur?

If you want to know whether we're regularly deleting images from there currently – no, not that I know of. We're monitoring our Imgur bandwidth though, so if you plan to hijack us to create a life-long image uploading service, we may notice, so think again :)

It's obviously easy for us to query for the information "Which images were ever legetimately used", and Imgur has an easy-to-use API. In other words, if we become aware of larger scale abuse, or one of us has a few free cycles and wants to do this as a side project, or a newly hired programmer needs a "prove themselves" kind of project, or whatnot – then we're talking about a fairly limited amount of dev work that would be necessary to do some house cleaning in our Imgur account, and suddenly your images are gone.

So the most important part in preventing abuse here is actually the common sense of the potential abuser.

1. Is this behavior expected?

Expected as in "You should do this" – obviously not. Expected as in "we know it's possible" – yes.

2. Does it not matter to Stack Exchange and Imgur?

It's pretty much impossible to prevent this 100%, at least up front.

However, hijacking our Imgur account poses a risk for the uploader: If they use their own regular Imgur account to upload images, they retain control over the image, and the one-hit-per-six-month threshold is trivially held for someone wanting to prevent deletion.

If, on the other hand, they use ours, that means they totally relinquish control of the image file, including the possibility to prevent deletion. If we decide to go delete an image that has never appeared in a post or chat message, they can hardly make an argument against us removing it. And they actually won't know until it's too late.

3. Is there already an active process keeping irrelevant i.stack.imgur.com images out of Imgur?

If you want to know whether we're regularly deleting images from there currently – no, not that I know of. We're monitoring our Imgur bandwidth though, so if you plan to hijack us to create a life-long image uploading service, we may notice, so think again :)

It's obviously easy for us to query for the information "Which images were ever legitimately used", and Imgur has an easy-to-use API. In other words, if we become aware of larger scale abuse, or one of us has a few free cycles and wants to do this as a side project, or a newly hired programmer needs a "prove themselves" kind of project, or whatnot – then we're talking about a fairly limited amount of dev work that would be necessary to do some house cleaning in our Imgur account, and suddenly your images are gone.

So the most important part in preventing abuse here is actually the common sense of the potential abuser.