I'm not sure if the correlation is 100%, but you're right in that it will definitely work if "save image as" is available, and when it isn't, it'll be uncertain. I think there used to be code that looked for images more agressively so it would work more often, but then it started making virtually everything downloadable (page backgrounds, buttons, etc), which was horribly annoying too. 10/10 very fair review. :)