Crop face and made it texture for 3d model

hi guys so i tried to make an app where user can take photos, crop their face and put the cropped image into an object face

i already tried to use getpixels and setpixels, but it not working correctly, i seems can't get the exact shape to crop, since the getpixels example using rect

i already tried modified it as good as i can, but still i can't get it to work

can anyone help me with this?, or at least show me the rope >,<

thank you in advance, i'm a noob programmer

i'm using C# btw