The head texture slot in the Textures part of the shop? That's a 128x128 head, or the lowest quality. In the actual shop and on the Markets there are two other sizes, 256x256 and 512x512.
If you bought the slot in the Textures section, you'll see "Choose File" in the slot labeled "Head Texture". Upload the flat (flat version of the head as made by the artist) using that.
You can also find the head texture item (not the texture) in your deactivated inventory and upload it there by hitting 'Edit'.
If not, then find the 'Textures' part of the Inventory category in the shop.