I'm new at this myself but I think what you may be looking for is the "CollisionPolygon2D" node for your sprite.
If you add this as a child to the sprite node that you've imported into the game and draw the collision shape around your sprite with CollisionPolygon2D, you should get something like what you're looking for.
As for using the .png file as the collision shape itself via code, that's quite a bit more technical (as far as I currently know); but someone else here may be able to help you with that :)
Here's a guide I found on Godot Docs that has more information.
Hope that helps and good luck!