I tried a few of the board icon mods to see if they worked with 2.1, and no success. So after looking at this thread...http://www.simplemachines.org/community/index.php?topic=530523.0
...I get how to change board icons for the whole forum, but how would one be able to use individual "boardicons.png" files for each board? I found it in the CSS as mentioned in the above thread, but couldn't track down anything related to icons anywhere else.
I believe that you would have to modify code , not just css
but here is the CSS....
div.icon span.board_off {
background-position: 0 -45px;
}
div.icon span {
background: url("../images/boardicons.png") no-repeat scroll 0 0 rgba(0, 0, 0, 0);
display: inline-block;
height: 45px;
width: 45px;
}