If you are just trying to use a different .bmp in place of the default mIRC toolbar buttons then right mouse click on the toolbar next to the current button (a blank space) and in the menu that pops up goto Buttons > Select and navigate to the directory that you have the new icon .bmp image at and select it. This will replace the default icon set in mIRC. If you are trying to replace the entire toolbar a suggested above you will need to either download and use the .dll's that make this possible or download a addon that will do this all for you.
