It can be customised by changing the file linux_logo.h in include/linux directory. It is a C header file, and pretty hard to change by hand, however there is a plugin available for Gimp from http://registry.gimp.org/detailview.phtml?plugin=Linux+Logo that will create one for you. All you need is a picture 80 pixels in height and width, with less than 224 colours. You can either let Gimp create the three varieties (2, 16, 224 colours), or create them yourself and use them with the plug-in. It will ask you where you want to store the file, and if you are game you can put it into ($SRCDIR)/include/linux/linux_logo.h. Once that is finished all you need to do is recompile the kernel as usual, reboot, and if framebuffer is working, you will see your new logo upon bootup.