Apple has a great article over on their support site on how to display an agreement window where users must read and click Accept before logging into the computer. This is really useful for people who manage lots of Macs in a school or company with an “Acceptable Use Policy” that all users must agree to.
To do this, just create a file in TextEdit that contains whatever text you want to display. You can embed images and use whatever text styling you want. Once you are done, save the file with the name PolicyBanner in the Security folder in the Library in Macintosh HD.