Here's another way of customizing the SAP Enterprise's Portal's Logon Page. This method seems to be very simple and straight forward, but I wasn't able to implement the steps as I run one of the latest versions of EP NW2004s Portal.
This process of customization works on all SPs of EP6 though ;)
So here are the steps:
1. Place your new image files here:
2) Log on to the Portal as administrator and go here:
System Administration -> System Configuration -> UM Configuration.
3) Click on the "Direct Editing" tab.
4) Change the parameter:
5) Change the parameter:
6) Click "Save all changes"
7) Restart the Portal