SCSS拡張のCompassで、background: url(...);の後ろにパラメーターを付ける ref: http://qiita.com/items/3180

file0.txt

compass watch --sass-dir=sass --css-dir=css --images-dir=images

file1.txt

p {
  background: image_url('cats.png') no-repeat;
}

file2.txt

p {
  background: url('/images/cats.png?1331742581') no-repeat;
}

file3.txt

@import "compass"; /* @include border-radius(2em) のために必要 */
p {
  height: 50em;
  @include border-radius(2em);
  background: image_url('cats.png') no-repeat;
}

file4.txt

p {
  height: 50em;
  -webkit-border-radius: 2em;
  -moz-border-radius: 2em;
  -ms-border-radius: 2em;
  -o-border-radius: 2em;
  border-radius: 2em;
  background: url('/images/cats.png?1331742581') no-repeat;
}