To activate this feature:

1) Create your own images (The file extension must be .png)
2) Save image file as: forum_icon_X.png where X is the forum ID (you can find this browsing the Forums Index Page or via CP )
3) Upload these images to: /eebootstrap/images/forum_icons/ (this directory contains 12 default icons 64px X 64px, you must overwrite these files)
4) Open /eebootstrap/images/assets/js/eeBootstrap.js at line 14
5) Change: var show_forum_icon = false; to: var show_forum_icon = true;