When Photo to Movie scales down a photo from it's full size, it must choose which pixels from the original image to use in the final scaled image. As Photo to Movie creates motion by smoothly moving and scaling the key frames, it is possible that a bright pixel in the source image will be chosen on one frame but not on the next frame. This leads to a flickering/flashing effect (the bright pixel will be included every other frame, leading to flashing).
The flickering problem is most pronounced on slow movement; photos displayed with fast motion (i.e. key frames are wide apart) do not have as much of a problem since the problematic bright pixel will be separated by several pixels in the final movie, so the eye won't pick out any flickering.
The solution in Photo to Movie is to perform processing to make the selection of pixels in the original image consistent from frame to frame. It does this by blurring the original photo just enough to blur out a pesky bright pixel, but (ideally) not enough to make the photo look blurry.
To adjust the adaptive blur settings
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Select one or more photos in the timeline.
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Switch to the Photo tab in the inspector.
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Adjust the adaptive blur setting in the menu.
The default setting is Medium.