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.
With the Contrast setting you can convert your full color picture to a pure black and white picture and that is handy!
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.