In the long run, it should be possible for ap_pipe (or any other pipeline) to automatically determine the correct templates for image differencing of a particular field. However, this capability has not yet been written.
As a workaround, ap_verify should add a command-line argument that lets the user specify the template to use with a particular visit. For consistency with ImageDifferenceTask, the argument should be named --templateId and take a (list of?) dataId that contain only a visit ID.