NO! NO! NO! YOU'RE DOING IT WRONG!!! but don't worry... Uncle Mugen is here to help!
Let's see... where should we start... first of all a renderer for sketchup is optimized to mimic real life which is not what you wanted for your BG-CG which I assume you wanted to make it look more hand drawn... right? The correct way is to use the renderer as a way to generate shadows and highlights instead of doing it the hard way and manually shading them...
Heres how I do it... it's not perfect but this should be help you find that right spot... at this point I'll assume you already have your model prepared, properly textured and got that perfect spot...
Go to sketchup and go to window > shadows and turn on the shadows then turn off show shadows but check the, use sun for shading and put the time at 12:00NN so the sun is directly above. Adjust the slider and lower the light settings and increase the dark settings until there is not much difference between the light and dark spots. Go to view > edge style and turn off edges. Export the scene as texture.png
Next... go back to window > shadow and turn on the show shadow and adjust the sliders so till you can have that perfect shadow you wanted. Then using your renderer, go render it in clay mode. When you say clay mode, you render without textures so you'll end up with a textureless image with proper shadow and light behavior... Export the scene as shadow.png
Next, hide the shadow window and go to view > edges > display edges to show the lines. Then go to view > face style > hidden lines to hide everything except the lines. Now go to windows > styles and click the edit tab... remove the sky and change the BG to pure white... Export this scene as lineart.png
Now go to photochop 7 and open the file texture.png... adjust the colors and make sure to blur out severely sharp details. The open the file shadow.png and put it above the thexture.png... set this layer to multiply and reduce the opacity till satisfied... then import the file lineart.png and put it above shadow.png set this layer to multiply and reduce the opacity till satisfied... then create a new layer above the lineart.png and manually draw some more highlights like the sharp corners of the cabinet being struck by light and the light from outside... add some details like stickpads and magnets or other stuff...
There!... Total time 30 minutes no kidding... cheap but quick... Lol!...
I seriously should make a more detailed tutorial for this someday...
"POOF" (Disappears)