When user 'touches' pixels with the mouse cursor (or touch on mobile) those pixels start to 'fall'. This will work best if the image is an isolated shape on a solid monochrome background.
User can export the whole animation to GIPHY. To limit the filesize a specific frame interval could be used (for example every 10 frames a screenshot).
Optionally: User draws a pixel mask on top of the uploaded image, to indicate which pixels will 'fall' when touched by the mouse cursor. This might be necessary when the shape to fall is not isolated from the background but part of say a landscape scenery.