Hi,
the best option is to include the base64 version of the image in CSS. Then you are completelly independent of the platform.
in the CSS style, instad of url paste the base64 encoded version:
background-image:url('data:image/gif;base64,<the base 64 code for the image>');
You can make the BASE64 encoding via the web page
Base64 Image Encoder - there you can upload any image and get the corresponding CSS content.
from the performance perspective, if you use the base64 version for many images in css try to have only one css class and assign those class to many components - just to avoid sending many bytes again and again to the client.
Karol