Some help about masks

I have found this tutorial and I want to include it into my image gallery:


what is my concern is how can I know the precise moment the image has been completely revealed so I can unmask it, and remove the masks from the stage and nullify them?
(and stop the timer and remove enter frame as well)

(bear in mind that image will not (neccesarilly) cover the whole stage in my case)

I appreciate any help…