Hi Mouhamad -
That's right. Applying a mask to a video applies it to the whole video.
You've already done the hard work... if you drop all your screenshots in the timeline and set them to 00:00:00:01 duration, then apply the matching masks to each frame, (then pop in whatever it was you wanted to replace those spaces with) you should be able to produce your 2 second clip.
Cheers - Tony
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