ok,
first you have to use the EXT3 version of WDLXTV. for the flash version wou will have no permission to change files.
Once you run the EXT3 version connect to your device though webends file manager or better connect through ftp.
Go to root/osd/image/.. and replace the welcome_logo.png with one of your choice with the same image dimensions.
Thats it.
For changing just the screensaver logo reseach for a command inside the S00custom option file. In that case is doesnt matter which version (EXT3, flash) of WDLXTV you are using.
cheers