Control the speed by changing the duration of the behavior.
Trim the duration of the still image to the duration of the behavior or it will "pop" change sizes. (You can also set an offset top stop the size change, but it's better to make them both the same length.)
Jim is right about how to control the duration of the behavior, but you could still find some problems adjusting the "speed" of your movement. You're dealing with more of the perceived magnitude of the effect than the speed of it. Set your grow/shrink behavior to the length you want your picture to be, and adjust the amount of growth or shrinking you want until the "speed" of the effect feels right.