The image you use is going to be on the network somewhere, quite possibly it comes from the same server as the CSS and the HTML. Although it could conceivably come from some other site. You need to provide the URL which points to that image, which you would do just the same as if you used an tag in the HTML.
I don't know what a "base 64 image" is. Normally you'd use a JPG or some other image type supported by your browser.