I found out how to run this (here, possibly) with:
rundll32.exe C:WINDOWSSystem32shimgvw.dll,ImageView_Fullscreen
which brings up an empty instance into which you can drag image files
I can also append a filepath to the incantation to load a specific image.
What I can't seem to do is create a shortcut that will accept a dropped file
I've tried various combinations of %1 and "%1" as arguments but nothing at all happens.
Is this just not possibble, or am I missing something?
I know it's easy enough to r-click and 'Open with...' if Image and fax viewer is not the default app for the file, but it's one of those 'because it's there' things