This week's book giveaway is in the Programmer Certification forum. We're giving away four copies of OCP Oracle Certified Professional Java SE 11 Programmer I Study Guide: Exam 1Z0-815 and have Jeanne Boyarsky & Scott Selikoff on-line! See this thread for details.
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.