closes #439

This PR provides more detailed conditions for when an `img` element is mapped to the `img` role. 

This was originally brought up to address what should happen when an `img` lacks or has been given an `src` attribute set to the empty string, as HTML defines such instances as "representing nothing".  However, in trying to just add in that condition, it made me realize the mappings were lacking additional nuance to how the element is exposed re: the existence of alternative naming mechanisms besides just `alt`.