its more of a support question but.
go to you're inventory and search the head tex. click on edit and wait till the page loads. you see a bar. click on it and you get a popup with you're files. select the head, hit upload image and login to the game. wait till it downloaded you and restart, done! you have a headtexture.
Option number 2: Go to torishop and look at the left side. near the active and deactive there is another tab called Textures. go there, click the bar that fits the item you want to put a texture on. and upload the image. again wait till it downloaded you and restart.
Good luck bro.