This plugin allows you to insert "Scrolling Overlays" into your posts and pages. 

A Scrolling Overlay is a view that appears, covering the whole screen, presenting some sort of media.
It displays at the conventional time- when it's scrollheight is in the viewport. 

To create an overlay, go to the Scrolling Overlays menu page. Then you can generate 
shortcodes by selecting images, videos, or inserting a Youtube URL. These shortcodes
are then placed directly into a post.