BackGround Images:

Background images can be pre-loaded into WCS before rendering. A directory
 (it can be anywhere on the system and have any name) should be designated
 in the "Settings Editor" window, "Pre-Processing" page.

If there are any numbers in the file names, they must correspond with the 
 frames to be rendered. If there are two numbers or more in the name
 (e.g. "MyImage1.27.iff") only the last number must match frame numbers.
 In the example here the file would only be used for frame 27. The frame
 number can be padded with zeros.
