This extension shows a Google Image search result for the current page
This will look for the background image in the Div you right click on. It usually works for slideshows. Actual background images may or may not be gotten. It should work. I'll probably won't update this until things break for me so use it as you like. Just right-click the image you want.
source can be found here.
https://github.com/ibsusu/BackgroundImage-Search