Im my project I have a main SASS file in the root. I also have a folder with _other.sass and icon.png in it.
Since all the compiled code will be included in your final
*.css file, all urls must be relative to that resulting
*.css file. In the code you have posted, the url is assigned as regular css code, so it wont be transformed in any way.
If you would use compass, what is a great extension to sass, you can use the provided functions, f. i. those to generate the needed urls. With compass you can configure all the base paths and the stuff alike, sass itself wont do that for you.