Not being able to click "back" out of a POST isn't broken, in my opinion.
Basically, if the "back" button behaves differently than it would have behaved in a functionally similar web app in 1998, it's broken. Web apps in 1998 didn't let you click "back" out of a POST.
In 1998, wouldn't a well done POST have issued a 3xx redirect? in which case, going ba k would bring you back to the form or originating page, right through the POST. In which case, the browser may still show the now-deleted image until the user explicitly decides to refresh the page. IIRC the old Opera browser was good about this, to the point of remembering dynamic DOM content as it was.
Basically, if the "back" button behaves differently than it would have behaved in a functionally similar web app in 1998, it's broken. Web apps in 1998 didn't let you click "back" out of a POST.