Also I like to customize my icons through reshacker
I wont comment on 'reshacker' but, it all depends on what icons you are looking to change..
You can change the MAIN mIRC.exe logo simply by creating a shortcut to it and changing the logo of the shortcut which is not only easy, but legal.
If you want to change the taskbar icon you can do so via the mIRC options panel.. ALT+O - Display - Options - Tray (v6.16)
If you want to change the icons in your mIRC toolbar, you can do so by simply right clicking on the bar and loading a bmp image file over the existing icons.
If you want to change the icons displayed in your switchbar, you will require some sort of dll that you can more than likely find from one of the many sites dedicated to mIRC, try, mIRCscripts.org
I would not recommend you change the icons via any other means other than either via the options mIRC provides (in options settings/ect) or via a .DLL file.