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Any sequence of images (for example, a PICT and BMP sequence) that you import directly into a Flash document is imported as successive keyframes of the current layer.
So name your images in order. drop them directly on the stage, as above, it will create as many keyframes as needed.
Go to File->export, select .avi or .mov as the export format (don't know why .flv is not there.) Test the movie, then run the Adobe Video Encoder on it to create an .flv for re-use in a Flash player.