Post the pic you want changed into a model and I'll tell you what you need to do. I need to see what you're working with inorder to give an intelligent sounding response. I think I kno what you need to do.
It would be a matter of having pictures on planes of a side , front and a top view of the object you want to sculpt/build in 3d.
Having accurate pictures is crucial. Differences in height or with in the drawing can cause nothing but headaches and wasted time
Once you have those placed you simply draw/trace polygons over top of the drawings in a logical manner placing them as you go using the top and side views as a guide.
I can't really speak to Blender specifically as I am a Lightwave user but the result and method is probably very similar in most apps.