You could take it easy and keep your frames in your 50 files: load them and define a fixed sequence (an Array?). Then add/remove them calculating the elapsed time with the ENTER_FRAME + getTimer method that @singmajesty is suggesting.
Add/removeChild is the simplest way to do it, otherwise you can add all frames and set them visible/invisible.
You should compare the efficiency of the 2 techniques: is it faster to remove a frame and add another one or to check 50 images to see which one must be rendered? If you have a single animation on screen I think the difference is unnoticeable.
If you are cool and want to be modern, create an atlas (a single image containing all your frames) and use spritesheet library or even better tilemaps as @singmajesty suggested, with the mapsheet library, that should be the most efficient render method for you.