What does ‘mask’ from imageStim do exactly? (Does it make parts of the image transparent, or is it like an overlay?) Why does the mask have to be a square? Is there a better way to create an overlay for an image to be displayed in a window? (I want to only show parts of the image, but through ‘holes’).
That sounds like you want an Aperture component, these let you obscure stimuli save for a specific area