It’s too easy: replace the indicated file by yours in the three directories indicated.
To change the default image used in the Vibe login window, a grey bar with the text “Novell Vibe”, with whatever is more appropriate for your site, the admin has to change one file in three (don’t ask) separate directories. I recommend you copy or rename the original for safe keeping.
The file to change is novell_graphic.png and it is found in the following directories (SUSE implementation) :
Note /31-05-2012-0900/ changes with the implementation or update (?)
Make your own image to the same dimensions of the original : 400 pixels wide by 60 pixels high. Although you might be able to get away with slightly different (if you look at the KaBlink and other relics in the respective directories).
You can see an example at : http://csdecso.on.ca. I have used a modified version of our branding background image for the login image.
Keep in mind that these files and directories tend to be updated / reset during updates. So double check ALL of your tweaks after each update and restore those which have disappeared.