As it is now, I think you need to update item.dat to acknowledge that you have a texture item, or else it won't show.
Go copy your custom folder and rename it to something like mytexturestest. Then go inside it, open item.dat with notepad or wordpad and replace certain lines with this code:
TEXBODY 0; 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 0 0 0 0 0 0
BMAPBODY 0; 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
TEXJOINT 0; 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
BMAPJOINT 0; 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
TEXTRAIL 0; 0 0 0 0
TEXGROUND 0; 0
Just find the lines similiar to this in .dat file, delete them and place the code in their place.
I took it from my own item.dat, since I have nearly all textures, it should work like a charm for you.
@edit
and of course then you must type /lp 0 mytexturetest ingame. Why make a "mytexturetest" instead of changing your item.dat you might ask - it's because whenever you open TB, it automatically downloads logged-in person's item.dat from the server, so the changes would be gone after restarting TB.
@edit2
The tutorial linked above by jmoran98 is rather outdated, for example TB now works with RLE compression as far as I know.
Last edited by JtanK; Sep 14, 2014 at 01:27 PM.