First, I created a few masks using the solution I turned in:
Figure 1: Mask of Tony Stark's face, not including his hair.
Figure 2: Mask of Black Widow's face.
Figure 3: Mask of The Hulk's face. I didn't end up using it because it was too hard to fit it on any of the other character's heads.
Originally, I thought I was going to put Hulk's face on Tony Stark, Tony Stark's face on Black Widow and Black Widow's face on the Hulk...That didn't work too well, mainly due to how much The Hulk's face varied compared to other normal people.
I then used Gimp to carve out the faces using the masks I created. However, for some reason, when I paste the carved faces back onto the image, Gimp wouldn't let me move their positions...so I ended up pasting them into Photoshop and Free-transformed/fit them onto other people's heads.