Currently, icons automatically provide just a simple presentation of how many replies are in the thread, to give a simple overview of all threads listed. If this is a new topic, there is a blank icon; if there are some replies, there is "common" icon, and there is "hot" icon if there are more than 2 pages in the thread.
Having the feature you suggest, would mean we need:
- an extra field in database (to store the name of the icon)
- an extra field of the form (to provide selection of an icon)
- an extra folder to store physical icon files (+ all files will increase the size of the package)
- an extra code to handle icon's selection upon adding, editing or displaying the topic.
Now if we think carefully - what kind of profit this feature would provide to take so many resources to be in? ;-)