If you were to buy a new head from someone, how would you be able to equip it?

You need to buy "head texture" from the shop.

When you talk about a new head I imagine you mean the image?

To wear this you first must purchase the head texture uploader from the Torishop, this costs 2,000 TC to purchase. Go here...

Once you have purchased this, you then need to get a flat image of the head you want to wear, if you head back to Torishop - textures. You can upload it there.