In order to do this (if I'm correct) you would need to do these things - in order:
1. BACKUP your forum images folder - that way if there are any mistakes, its easy enough just to restore quickly

2. Look at the forum images within that folder. Most of the ones you
want to change have unique names and are created as a .png file and a
.gif file - so you would have 2 of each but ending with either .png or
.gif
How to do it
Say you want to have a new icon as in poll - open the new poll icon in a photo editing program (not paint) and then save it as:
poll.png and then as poll.gif
The only issues I can see though is the flame, pin and padlock symbols are like overlays that will appear on top of the image, so some tinkering and trial and error will be the way forward. Also pay attention to image sizes as it is pointless creating an image size that is bigger than the original sizes
I hope that helps but obviously Bruce who designed the software would be the best one to answer your post