What is needed for this tutorial is a graphic tablet of any brand is helpfull; a scanner, or you could simply start by drawing directly in the program of your choice (ex: photoshop)

1) Scan the picture at 300 DPI. Greyscale is enough and you don't have to clean it up, remember we want to paint grunge-like. If you'd like to sketch directly in photoshop then skip this step *obviously ;) and continue with step 2.(_view image_)

2) You now should have either a scan or digital sketch to work with. If you didn't scan at greyscale-mode, then set the mode from RGB to greyscale, because we will begin painting at greyscale. This is easier and cleaner than painting in RGB, you may accidentially use brownish colors instead of greys and that's not so good.(_view image_)

3) Set the sketch layer to multiply and create two layers underneath it. The bottom layer will be used from the background. Select the paintbucket-tool and fill it with a grey tone. Don't use a dark grey or the whole picture may end up looking too dark. Set the second layer to multiply and select the airbrush (mode multiply) at about 50% precent opacity and 0% percent hardness and don't forget to activate the tablet settings (pen-pressure). Use a darket grey and start painting values to give the figure depth and shape, remember to pick a light-source and stick with, After basic shading with the airbrush, use others to add textures.(_view image_)

4) You now should have a greyscale version. I call this state "skeleton", not because I'm a cool
d arkartist, I just call it that. A skeleton is a basic shape after all and that's what your drawing is
right now, too. Now we will attend to the background - this step is a really fun part, because you can go wild with preset brushes :D. Use what ever fits your fancy! Just create an interesting texture. Sorry there is no recipe to do this...(_view image_)

5) Good, we're almost done. Now that you're pleased with the background and the overall value of your drawing, we continue with setting the picture to RGB-mode. Don't flatten the picture yet. You most certainly will have to rework parts of the background. Now that the picture is in RGB-mode we can add color. Create two new layer on top of all your already existing layers and use the paintbucket-tool to fill it one with red, the other with yellow. Or any other color you think will work. Set the red layer to color burn and the yellow to overlay - suprise! The layer really brings the painting to life, don't you think?(_view image_)

6) The last step - cleaning up, changing parts, adding more textures and light-effects, be patient :).
Create a new layer and paint on it using only white and black. All done and satisfied? Good job :) Thanks for reading and remember, this way is an easy way to paint, but the program won't do all of the painting for you. The computer is a tool like any other art-tool and you have to know how to use it. (_view image_)