I like to use some constant to refer to the paths.

For example:

@PATH_ICONS: ../static/img/icons/;
@PATH_IMG: ../static/img/;

and pass to a mixin:

.button_icon (@img) {
background-image: url(@PATH_ICONS@img);
}

I try too with:

url(../static/img/icons/@img);

and fail. Only way is pass the whole path as param:

url(@img);