This is a handset feature, not a system wide feature. This means that each handset needs to be done individually,
You need a TFTP server on site ( Pumpkin tested )
Image:
Prepare your image , must be called WallPaper.BMP
TFTP server config:
Place >BMP file in the TFTP root directory, allow all requests
On DT900 handset
With handpiece down:
Press: