Is it like a decal on your head?

A head avatar shows your head texture under your score while fighting ingame.

no, a head "decal" would be the head texture item
the head avatar item is something used ingame to show your head under your name spinning while you fight

Where do you get stuff to customize the picture on your head, like a halo face or something? Is that a texture? Becuase all I see is default and bumpmap. :/ Where do you get the cool faces?

you buy a head texture item.
since they are sold out in shop, you can request one in the items forum
it will probably cost around 20k

First, you must by the texture item. This allows you to upload images on to your head. The cool heads you see are images made by other Toribash users.

You can purchase body textures here and their bumpmaps here.
Then all you need is this to have custom textures on yourself.