I'm fairly new to Stencyl, but deep into my 2nd game, but since nobody has answered yet, I'll try. First of all I recommend doing a couple practice games that are tutorials, that are similar to what you are trying to do.
I would use Photoshop (expensive) or Gimp (free) to make at least two images of your actor, one normal and one with a punch thrown, or grab 2 images from the internet if you're not worried about copyrights. Or get free images in the packs offered here. More images (frames) make it smoother, but there are of course disadvantages as well. Import the 2 images into one actor in Stencyl, experiment and adjust the frame timing to your liking. You can customize the collision zone of each frame I believe, or maybe it's just the whole actor. Been awhile since I did that.
You could trigger your animation various ways, like on a mouse click event, but you probably want to trigger it with a collision event. I am rusty on the collisions, it's been a couple years. My current game has none. You can start and stop your animation or switch to certain frames, see attached. You can make it loop or not loop.
I realize I may not be much help here, but I try since the people here are nice and usually help me. If I could offer one piece of advice, it's do the tutorial games, or at least the pertinent ones. I often don't want to, but I'm always glad I did. It definitely saves time overall.