I ment "new to After Effects", sorry
For a mask path to work as a motion path you cannot move, rotate or scale the layer you want to use or the path will not line up. You can get away with rotation by adding a null to the timeline, parenting the layer with the motion path to the null, rotating the null then releasing the parenting. Maybe this will help:
I would draw the path on another layer -say, an empty solid layer, as a mask. Then cut that path and paste it into the PNG's position property. This might be happening due to a rotation of your imported PNG; the path doesn't "know" which way is up, so it keeps the image's "up" (which is really the right side of the screen in your screenshot).
You can alternatively rotate the path before cutting it, by double clicking on it for the transformation box to come up, then rotate it by placing the mouse close to one of the corners (until you see the rotate cursor), and rotate it 90° clockwise to compensate for that rotation you're seeing. THEN copy the path and paste it into the position.