Do you have some tool like paint.net or photoshop? To cut off the background of a sprite, you need to make the background transparent.
To fit a collision polygon to sprite:
Edit animations>Click set collision polygon icon>Right click sprite>Guess polygon shape
But, I generally don't recommend setting a custom collision polygon to a sprite; especially if it's shape is irregular.
Instead, pin an invisible sprite (a sprite that's completely transparent) to the general overall shape of a sprite (or have it mirror the object's x/y co-ords) and use that for condition chekcing with collisions.