Is there any way that I can have Word choose to include a particular picture
based on a value in a mailmerge field?

I have seen that there is a LINK instruction but I can't find any good
examples of how to use it.

What I want to achieve is for PictureA to be the background to a particular
half page, but if there is a value in a particular merge field, for the
background to become PictureB?