Hi Scirra people!
On elaborate animations not all the frames are visible for exactly the same time.
For istance:
<img src="http://www.petesqbsite.com/sections/tutorials/tuts/tsugumo/119.gif">
the frame where the kick is fully streched lasts little longer than other intermediate frames. That gives a sense of realism on the animation.
It would be a good add the possibility to specify not only the overall sprite's animation speed, but also the single frame duration.