If you want other people to be able to see your head texture, you need to buy the item.
http://forum.toribash.com/tori_shop_item.php?id=1337
^^that is the cheapest head texture available in the torishop. It costs 2000TC and it allows for a 128x128 size texture.
Once you have bough it, you can upload the texture you made for it here:
http://forum.toribash.com/tori_textures.php
If you just want to see what your texture looks like without buying it, you can do so by saving it in your customs folder.
C:\Games\Toribash-3.9\custom\wiktor1402
When you save the texture you need to use the correct file name, "head.tga". The next time you go in-game you will see it on your tori.
You can also use uke to test texts on. Simply put the textures here.
C:\Games\Toribash-3.9\custom\uke
To get all the file names you can use the command /dl uke. Uke has a complete texture set, which after you download it, you will be able edit and try out texturing other body parts.