mIRC uses a custom drawn static control for channel windows etc, The images would have to be drawn on by the DLL along with the text.
This is where it get's tricky not to override mIRC's display and scripting features. And why a novel and unique approach is required, Which is really why I am asking here, I can't get my head around the best way to approach it. Infact I am starting to think this is not possible.
