To animate something in osu you need to put each frame as its seperate image. For e.g. menu-back this would be (note that the counting starts at 0):
menu-back-0@2x
menu-back-1@2x
menu-back-2@2x
....
Normally all animations play all frames every second, however, you can set a framerate in the skin.ini with AnimationFramerate: which will affect all animations (besides hitbursts and hitcircleoverlay)