http://blog.teamtreehouse.com/the-ecstasy-and-agony-of-compass-sprite-generation-part-1
http://blog.teamtreehouse.com/the-ecstasy-and-agony-of-compass-sprite-generation-part-2
$my-sprites: sprite-map( "my-sprites/*.png", $spacing: 5px);
$height: image-height(sprite-file($my-sprites, $file-name));
$width: image-width(sprite-file($my-sprites, $file-name));
http://compass-style.org/reference/compass/utilities/sprites/base/#mixin-sprite-dimensions