The images are relative to the HTML document, but inside an email client, this stops working.
There are two ways of including images in HTML emails:
1) Embed them. I'd suggest using Apache Commons Email for doing this. However, I think this won't work with background images.
2) Use absolute URLs to some web server. However, most clients block these images by default. You can't work around this, it's a client setting.
So I'm afraid what you want isn't going to work the way you want.
You should consider reworking the HTML to use IMG tags instead of background URLs, and then embedding the images.