Creating fractal animations

I want to create animations based on generated fractals
(e.g. a movie constructed from Julia sets as the initial
point travels over a sequence of values).

Creating the images is no problem; what I need help with
is taking individual images and creating an mpeg (or some other
movie format).

Has anyone done this? Is there some preferrably free and
easy to use software or library that will help me do this?

Thanks for any help,
-ra
.