Yes, you would effectivly be making your background into a video by having an image file of each frame.

The original idea behind flash was to be able to create motion and 'movies' with much smaller file sizes, this was done by storing everything as vectors rather than images. These days internet speeds are fast enough to be able to handle video with ease. Video also makes it much easier to run complex scenes as it takes less power to decode.

There is always a compromise between speed and file size (video will be bigger files, but usually easier to run)