Google's John Mueller said this morning in google webmaster mark that Google Image search does not index and rank images from CSS background code. He said if you want your images to rank in Google Image search then you best use normal image tag with the source attribute pointing at the image.
Check google webmasters for more details.
My website seming lighting official page was once set the pictures in the background. It's not good for the picttures index, i changed the code to img and it's getting better now.