Where can I get a texture template that show the position of the joints ?

The joint textures are the same with the head texture.

no. I mean like a tricep texture, but the template show the covered with joint part when we put it in our tori.

There are none yet, sofar as i know.
may be some usermade ones that Im not aware of yet.
nblx or 4edrick may have some.

apart from that, most people work it by eye.

Would this be really useful?

Using any of the 3d rigs this would be pretty easy for someone to make.

There is one on the nblx texture tutorial, it is by 4edrick. He owns one as well as goodbox, I can't find the link right now but I might be able to get it for you later on.

It's useful when you want to texture below the joints with bones and such. Have seen some people with such textures and I find it pretty nifty.

I have used these. I have templates only for torso. Can't remember where I found these but they are pretty good.