Would like to show a photo through a polygon shape

[Richmond]

I have just had a bit of fun that may help towards a solution:

I imported an image into a stack and then made a 5-sided regular polygon 
graphic on top of it.

I made the polygon graphic transparent (Polly Gone !!!!)

The I started messing around with blending/ink (that discrepancy with 
naming between the prefs palette
and scripting can be a bit misleading) and found that

blendDstAtop  and  blendDstIn  (who invented those awful names?) make 
the area inwith the rect of the regular
polygon graphic but outwith the polygon graphic itself black, producing 
a frame (of sorts).

The snag is that all the image on the layer below the polygon graphic 
that is not masked by the black area is visible.

Richmond.