Portrait and sprite mods don't go in the Content folder. Character sprites go in Content\Characters and portrait mods go in Content\Portraits.
@Dr.Drub There may be something wrong with the .yaml file. I'd suggest unpacking a default Leah portrait and sprite sheet and packing their .yaml files with your pngs. Or you can upload both of your modified files in a zip here and I can test it myself.
@Dr.Drub I unpacked your xnbs and repacked them with the default .yaml files and they worked fine for me. Hopefully it solves your problem.
