I have managed to do this here is my code which of course only relates to my Moodle set up, but will work in any Moodle install providing you change the image file names.
CSS:
td.category a[href="http://localhost/moodle199/course/category.php?id=1"],
td.category a[href="http://localhost/moodle199/course/category.php?id=5"],
td.category a[href="http://localhost/moodle199/course/category.php?id=11"],
td.category a[href="http://localhost/moodle199/course/category.php?id=17"]{
position: relative;
right: 20px;
line-height: 1.5em;
padding-bottom: 10px;
padding-left: 20px;
vertical-align: middle;
font-size: 1.5em;
background-position: left top;right: 20px;
line-height: 1.5em;
padding-bottom: 10px;
padding-left: 20px;
vertical-align: middle;
font-size: 1.5em;
background-repeat: no-repeat;
}td.category a[href="http://localhost/moodle199/course/category.php?id=1"]{
background-image: url(pix/themes.jpg); }
td.category a[href="http://localhost/moodle199/course/category.php?id=5"]{background-image: url(pix/design.jpg); }
td.category a[href="http://localhost/moodle199/course/category.php?id=11"]{background-image: url(pix/css.jpg); }
td.category a[href="http://localhost/moodle199/course/category.php?id=17"]{background-image: url(pix/xhtml.jpg); }
I hope this all goes to help you achieve your goal!
Mary
