Option to display an image's resolution in its tooltip

Suggested by [this reddit post.](https://www.reddit.com/r/Enhancement/comments/2jqkh9/is_there_a_way_to_see_the_image_resolution_when/)

I've tested it with pretty much every image expando possible and image albums (in Chrome).

Although a lot of people probably won't use this, it is a pretty lightweight change so I think it would still be worthwhile.