Google does not treat CSS content the same as that on page
Generally Google will only attempt to index content that is actually embedded within the page content associated with an appropriate tag such as
<img>. You can however attempt to force Google's hand by adding the path of the background image into a image sitemap.
Some Schema markups require more than just the standard itemprop
Valid Schema involves marking up actual content, not content used by a template. Using the itemprop image will require a mark up of URL which in this case you don't have.
CSS backgrounds are not considered page content
Your attempting something that shouldn't be done for various reasons, if the content is valuable to your users then the correct usage would be to include the content within the designed elements and not that using background url. General UI elements are useless for indexing in Google's image search, if you want the image to be indexed without the need of forcing Google's hand then there are several ways it can be done. A messy example would be to use position:fixed among many other ways this can be done... see my mess around fiddle as an example.