I've previously posted about this,
But I'm still having problem with my rotoscoped shot.
It turns out my matte is actually the wrong way round.
Can anyone tell me why the invert foreground / background option is greyed out?
Off the top of my head, no... but it doesn't really matter. Both Alpha and Luma Track Mattes have their inverted versions, so if you want your photo to show inside the figure, then select Alpha Invert instead of Alpha and you'll get the desired result.