Change to only scroll visible image - prevents image gaps; allows viewing tall images

Great updates. The attached prevents images from scrolling until visible, allowing tall images to be seen in entirety and fixing a visibility problem I had on http://corbiscrave.com.  Also fixes a problem where browsers suddenly resized-tall have a gap below the current image.  

The change moves "background-position" to only current element.  Since only visible slides move the reference to windowHeight goes away; this prevents a pop when entering the viewport. Let me know if screen shots are needed.  

Thanks for your help.