Easy: https://i.redd.it/dwi7jrmrwri21.jpg -> https://i.imgur.com/bQZXrAJ.png (Okay, but doesn't mask shadows around the feet correctly)
Medium: https://500px.com/photo/195143459/ximena-by-agata-serge -> https://i.imgur.com/kUdcvFo.png (Okay; it could have taken out a bit more background in some semitransparent sections of hair)
Hard: https://i.redd.it/i7fn0fy89ma21.jpg -> https://i.imgur.com/WGM67XL.png (Fooled with some false-positives, but still a good try)
Certainly not perfect, but definitely usable.