The page title corresponds, obviously, to the page you're on. Whether you've "left the page" is somewhat subjective in some cases, and depends on the mental model you build up. For example, Facebook's image viewer does not change the page title, because it is clearly a panel on top of the current page, whereas Flickr's image viewer does change the page title because the model is that you're navigating a new page associated with the image.
Given that the MediaViewer has a cross at the top right that dismisses it, and takes you back exactly to where you were, the mental model currently for MediaViewer is that MediaViewer is not a separate page, but is instead a panel that appears on top of the current page. Conceptually, I'm not leaving the article I'm reading, I'm simply enlarging one of the images in the article. As such, it's confusing and inconsistent with that model for to change the title of the page when you click on an image and open it in MediaViewer.