To be able to have a realistic fill most 3D engines allow for a realistic look throw the textures, if you didn't get that part read the tutorial on UV/Unwrap. In this tutorial the author is going to explain what texture does to the 3D model, what kind of results can you get and what kind of program to use to enhance the look of the texture.
In the tutorial the author is using Crasy Bump which is paid software but have a 30 days license, if you do a search you will find Awesome Bump which people claim that does the same and is free. I was not able to use Awesome Bump, because it only supports OpenGL 4.0 , my 3D graphic card only goes up to 3.2 GL. The key issue here is not the program, because there are several programs that due the same. The point is to understand how texture maps work. In last resort you can still do something similar in your 2D editor, in this kind of programs the difference is it facilitate the creation of texture maps, because you can see in real time the result, but after acquire some experience i can imagine that you can do that by experience in a 2D editor. Save you some time but is not dramatic if you can't find one.
Crasy Bump Paid but with 30 day trial - Crazybump.com
Awesome Bump free but only support new 3D cards - Awesomebump.besaba.com
Texture Maps Tutorial - Youtube.com