If you want to add textures to your tori, just buy one of the texture items (128, 256, 512) and upload images to them.
If you want to wear textures without owning a texture item, which is server sided and will only be shown to you, just find the image you want to wear, change the name to the specific body part you want to wear it as and add the extension .tga
Example: L_Bicep.tga
After that, enter you toribash custom folder (C: \Games\Toribash-4.1\custom), find your name, and paste the .tga file in there. But when you go in-game, make sure you do not /dl yourself, as that will reset it and you'll have to start again.