In the SkullOwner field you can type in a player's name or paste in a give command generated from http://minecraft-heads.com/custom/heads-generator. MCStacker will extract what it needs from it.

If entering only player names, the texture will display as Steve or Alex in the inventory if obtained with the give command. It will display properly once placed and then broken. For best results, you should enter a specific player's name into minecraft-heads.com and use the output from it instead of entering only a player's name directly into MCStacker.

Below are some heads you can use: MHF_Blaze, MHF_CaveSpider, MHF_Chicken, MHF_Cow, MHF_Enderman, MHF_Ghast, MHF_Golem, MHF_Herobrine, MHF_LavaSlime, MHF_MushroomCow, MHF_Ocelot, MHF_Pig, MHF_PigZombie, MHF_Sheep, MHF_Slime, MHF_Spider, MHF_Squid, MHF_Villager, MHF_Cactus, MHF_Chest, MHF_Melon, MHF_OakLog, MHF_Pumpkin, MHF_TNT, MHF_TNT2, MHF_ArrowUp, MHF_ArrowDown, MHF_ArrowLeft, MHF_ArrowRight, MHF_Exclamation, MHF_Question.