The concept arts constitute the essential graphic basis of a game project. It is with these drawings that the artistic director define the lines and gives indications to the people who will subsequently have to model, texture, animate…
David Revoy, This “painter and illustrator” made an excellent tutorial(1h:30 long) video on all the graphics work. Not only did he do it originally for the Blender Foundation, but in addition he merged it all to an 1hour and 30 minutes long video tutorial on Youtube. And especially: it uses only free tools and is based on Gimp and Alchemy for this Tutorial.
By the way, visit the site or DeviantArt page. He has worked as an art director at the Blender Foundation and you will recognize the art concepts from some of their videos. A talented guy who shares and teaches with as much talent as he draws:
http://www.davidrevoy.com / http://deevad.deviantart.com
And here is the video in question(in English, although it is French from the beginning, but very easy to follow)