hellogoodbye

Hi i have recently dlowladed Paint.net and thnk it's great! I am however facing a minor diffuculty with one my pictures. I have used the 'Lasso select' key to select my face and put it on my friend's face. what i wanted to know was how to soften the edges so that the edge of my face blends into the rest of my friends face? I have tried the tranparent gradient but it seems to make the entire picture transparent please help. Thank you =)