Edit: New link.
First of all download WoW Model Viewer. RapidShare: Easy Filehosting (Taken from anothere mmowned post)
when you open up WoW Model Viewer you should see somthing like this
on the left click the Tabs it shows you Items/ Characters / creatures, Etc.
Select the Item/ Character/ creature you want To get an Image from. ( i will be using a NE)
If you Selected a Character you can Dress it up on the right hand side.
now you have Dressed your CHaracter i will show you howto take a PIcture of it.
Firstly Click File>save screen Shot.
now save it to where ever you want. ( i use desktop)
now go to where you saved the File and double click on it.
This should come up.
now you Just Learned to Export an Image form Wow Model Viewer.
now you can use the image in signatures Avartars and all other things.
If you want to make great Avatars and Signatures Check out Roflcopterzz Guides how to make them in the Graphixs section.