To modify the chat mod icons in your wiki's Special:Chat, do this:
You should add this to your "MediaWiki:Chat.css" page:
.User.chat-mod .username:after, .User.staff .username:after {background-image:none !important;}
.user .username:after {right: 6px !important;}
/* Custom Chatmod Star */
.User.chat-mod .username:after {
content: url("http://images4.wikia.nocookie.net/__cb20110305043246/runescape/images/2/24/Prayer-icon.png");
}
That makes you have 
as your chatmod icon. Just change the url on the second last line to another image url to change the image.