The node icons are defined in your:
Admin CP -> Appearance -> Style Properties -> Node Icons
The default icons use a sprite image, but you can replace it with individual images if you want.
You can also create separate icons for each forum:
http://xenforo.com/community/threads/rc-1-custom-node-status-icons-read-unread.10886/
But in 1.1 you must also set the background-position unless you are using sprites that mirror the default node icons. Here is the code from that thread with the background-position added:
Rich (BB code):
/* custom status icons */
.node .node_7 .forumNodeInfo .nodeIcon, .node .node_7 .categoryForumNodeInfo .nodeIcon {
background-image: url("@imagePath/xenforo/widgets/forum-read-7.png");
background-position: 0px 0px;
}
.node .node_7 .forumNodeInfo.unread .nodeIcon, .node .node_7 .categoryForumNodeInfo.unread .nodeIcon {
background-image: url("@imagePath/xenforo/widgets/forum-unread-7.png");
background-position: 0px 0px;
}