Eddie this is the code you need for individual forum icons:

Code:
/* forum icon for forums with old posts Introductions */
.forum-list-container .forum-item[data-channel-id="16"] .cell-forum .icon {
    background: transparent url("images/icons/brd2.png") no-repeat;
  width: 48px; /* icon width */
height: 48px; /* icon height */

}
/* forum icon for forums with new posts Introductions */
.forum-list-container .forum-item.new[data-channel-id="16"] .cell-forum .icon {
    background: transparent url("images/icons/brd1.png") no-repeat;
  width: 48px; /* icon width */
height: 48px; /* icon height */
 
}
I would create a new file in your root for the icons, you need a new post icon, and an old post icon for each channel.

You will need firebug to be able to find out the data channel id

This will not work for sub forums