BootXChanger will allow you to change the boot image and background colour on Mac OS X 10.4, 10.5 and 10.6 This screenshot will explain how to use it:
If your image has transparency, it will be redrawn over the background colour before being applied, so save the original somewhere in case you want to change the background colour later.
To set an image as a boot image, open BootXChanger and drag it to the image well, and click Apply. You can also click the background colour to change it. You will then be asked for an administrator password, as the boot image is stored in a system file.