Have a variable in images path in Sass?

No need for a function:

$assetPath : "/assets/images";

...

body {
  margin: 0 auto;
  background: url(#{$assetPath}/site/background.jpg) repeat-x fixed 0 0;
  width: 100%; }

See the interpolation docs for details.

Have you tried the Interpolation syntax?

background: url(#{$get-path-to-assets}/site/background.jpg) repeat-x fixed 0 0;

Was searching around for an answer to the same question, but think I found a better solution: http://blog.grayghostvisuals.com/compass/image-url/

Basically, you can set your image path in config.rb and you use the image-url() helper

Adding something to the above correct answers. I am using netbeans IDE and it shows error while using url(#{$assetPath}/site/background.jpg) this method. It was just netbeans error and no error in sass compiling. But this error break code formatting in netbeans and code become ugly. But when I use it inside quotes like below, it show wonder!

url("#{$assetPath}/site/background.jpg")