OpacityMask for Custom Image

I have a list of cells which the user can assign images (transparent PNGs) to. I'd like to display an outline on each cell around the PNG when it's selected. I think this is achievable with an OpacityMask, but from what I've read and my limited use of them in the past, the mask data needs to be set in advance, which won't be possible due to the开发者_运维问答 user being able to change the image.