Use better HTML for images in sr_listings/sr_listings_slider

[CodyReichert]

Use an <img/> tag for the photo in [sr_listings] and [sr_listings_slider] instead of a <div>.

Also add an alt and title attribute to these images, to improve SEO.

[CodyReichert]

The only issue with this is that many people are probably targeting the old div elements in their custom CSS (ie, for a grid layout, someone may be targeting these elements).

Either we need to push a major version or have some backwards compatibility.

II'll just hang on to this one for now and likely merge it with other structural markup changes in a new major version.