1. Press Alt + P (Popups) then click view Menu Bar. The very first line you'll see 'Commands'. That's what you need to change.
2. If you have your own toolbar icons you can right click on the toolbar go to the Buttons menu and click 'Select'. You set the image you want and click ok.
Do a google search for mIRC toolbar buttons. You might be lucky!

I usually use
these.
-Andy