Here's a sample for my next picture. I'll give the full story when it's finished.

Basically, it takes place in a dank bathroom like the one in Silent Hill 4. Since I got the Line work and basic colors done, now I need to detail it. I was going to start with shading and actually used a picture of the SH4 bathroom as reference. However, I'm still unsure of how to shade.

The Program I use is GIMP. Any other program is out of the question, as GIMP is the only one I've used. So using GIMP, how would I detail this picture?