Doing some surfing on the net I found this (although it is already a bit older I still think it is good to post it here)
At www.legoengineering.com a new tool is posted.
With this tool you can easily create your own images to display on the NXT.
I downloaded the tool and did a little test.
Sure, you can draw and put characters in the display image, but the real power is in the import image function.
Then I did add some text and I'm ready to save my picture.
It automatically saves in the Pictures directory of the NXT software, for easy use later.
Now I can use the NXT-G software to upload my custom picture to the NXT.
Have fun Using this nice tool.
Download it at: LEGO engineering.