To: Tess
I read through that thread before I posted here. It was of no use the first time either. What I wanted was to create my own custom mask, not apply someone elses.
And I figured it out on my own, so I'll post the solution now. What I was doing (that didn't work) was trying to take an image, change it to 2 color (black/white) and set the background transparency. It looked like the other masks, but didn't act like the other masks.
What I should have done (and finally did), was take my 2 color image, select only the mask part (not the background) and paste as new image on a transparent canvas. Once saved it behaved correctly.