How to replace an image
[src$="/Themes/Blockland/images/IMAGE.gif"] { /* This is required to specify the image */
background-image: url('http://i.imgur.com/image.gif') !important; /* Actual Image */
width: 0 !important; /* Unknown why, but this must be 0 */
height: 12px !important; /* This is the image's actual height. */
padding-left: 12px !important; /* This is the image's width, for some reason this works, but the actual width style must be 0 */
}