you cant change it int eh bios, however you can change it in an xbe using a program called logo-x
that way, whenever you load an xbe with your custom logo in it (usually your dashbaord) it will show up, however it wont work with an x2 bios, as their logo with override it (so youll have to use an evox bios) and it wont work when booting games (unless you change the logo in those xbes as well)
also, i dont know where any tutorial is for this