Set up a structure something like:
/index.html
/img
/image1.png
/image2.png
/css
/styles.css
Move all your CSS rules into the external styles.css stylesheet.
Now, within the CSS, your image references are relative to the location of the stylesheet - so you can use relative URLs like background-image: url(../img/image1.png);
Finally, make sure that in your HTML code, you use an absolute URL to link your stylesheet - like:
<link type="text/css" rel="stylesheet" href="/css/styles.css" />
Using relative URLs within your CSS means you're free to move your stylesheets and background images into different folders - or even to a different domain or server - whilst the absolute URL /css/styles.css
in your HTML LINK tag won't be affected by mod_rewrite or anything else that affects your pages' apparent location on your server.