You can change the lock screen (in lock mode not curtain mode... effectively the same thing you can still control it oddly enough),
I don't know if you can get rid of the text (unless you changed the screen to white and kept the image size the area of the screen.).
Create a picture using a graphics program, such as AppleWorks.
Save the picture in PICT, TIFF, GIF, JPEG, or any other QuickTime-compatible static image format.
QuickTime-compatible movies or QuickTime VR objects cannot be used.
Name the picture "Lock Screen Picture".
Copy the "Lock Screen Picture" file to /Library/Preferences/ on the client computer.