For both Dave H and CuberToy:
As we have not got the documentation up and running yet here is a quick shot at it.
You will be editing the template for the theme you are using. As always – remember that if you edit one of the supplied themes then it will be overwritten on a subsequent update so it is recommended to make your own theme by copying the exiting one you are using.
So – to make this change to the forum view (list of topics) go to the templates folder of the thee and open the template for Forum View.
If you take a little time to read through it you will see how this constructs the forum view display. It is really pretty straightforward.
In this case the function that displays the topic icon is probably in the theme as:
sp_TopicIndexIcon('tagClass=spRowIcon spLeft');
each function has a pre-defined set of arguments that can be passed but you only need to pass one if you want to change the default. To add an alternative icon as requested then change the above to read:
sp_TopicIndexIcon('tagClass=spRowIcon spLeft&iconUnread=iconname.png');
Note the ampersand delimiter (just like WP template tag functions) and no spaces between arguments. You will, of course, have to supply the icon in the themes images folder and name it correctly in the argument above.