In Vista, menus have a "bar" on the left side for icons for each menu option. It would be nice to have the ability to include these in mIRC's popups. Obviously, mIRC would have to handle choosing whether or not to try displaying them based on OS or at least just not throw errors when the icons are included and the script is run in pre-Vista. That shouldn't really be an issue.
I think that the icons used should be accepted at least in ICO and JPG/GIF formats. With the ICO format, you could easily put all of the icons into a single file rather than including a lot of id individual icons. With the JPG/GIF format, it would allow people who don't know how to, or don't want to, create ICO files.