Here's the plan:

• We'll add an animation_rate property to MySprite. This is how many frames go through before the sprite automatically switches to the next costume.
• We'll need a counter for each sprite to keep track of how many frames have gone through
• In the sprites Do_Frame_Things function, we'll see if it's time to change costumes.
• To start the animation running, we just set animation_rate accordingly
• To stop the animation, we just set animation_rate to zero

Once done, an animated sprite becomes just as easy as an unanimated sprite (apart from creating the image).