to use custom icons with TB, you should define them in a file named batskin.xml available in the same folder as the main TB executable thebat.exe
I use this file myself to have a more clear icon for replied messages, because the default one is too hard to see. However I just use another existing overlay image, and not a completely new custom icon. To have completely new, trully custom icons, you'll have to modify the .png file that contains all the TB icons - not sure if it worths the effort. Anyway search ritlabs.com for 'batskin.xml', there was a file with instructions how to do all this, but unfortunately I don't keep it. Maybe some of the advanced users like Roelof can give another advice or a direct link to the zip with the neccessary files.