you must have the corresponding texture item, which can be bought either
here: http://forum.toribash.com/tori_shop.php?cid=54
or in the market: http://forum.toribash.com/tori_market.php

if you already have them, click shop, then go to your deactivated inventory.
after that, click [Activate] next to the item name.

in the list on the left, there'll be a section called ''inventory''
which consists of Market items, Activated, Deactivated, and, Textures

click Textures, and you should be redirected to this link:
http://forum.toribash.com/tori_textures.php

click browse, next to the texture item, search the image file through your computer, and upload it.

if it takes too long to get updated in-game, try pressing /dl caseiras
to download your custom folder manually.

hope it helped.