Im going to assume you mean a forum avatar.
If that is the case, go to your "User CP" just below the top left of the forum.
On the left side find "Edit Avatar" From there you can upload an image from your computer, or type in the image address of the avatar you want. If you do not have Toriprime or VIP you must use an avatar size of 100x100 or less.
If that is not the case and you want textures for your toribash player, you can buy the respective items in the torishop, or the market by searching their names. Once you have the items, you can either buy a texture set from the Art/Textures board, or make your own with image editing programs like GIMP or Photoshop. Then it's as simple as uploading them onto your texure item and activating in in your inventory.