This image was made for our EU cellulase project home page, and the hope was to produce soft drop shadows that looked right on the wood-texture background.
I used The Gimp, but the principles are the same for Adobe Photoshop.
The procedure was as follows, and more explicit details may follow when I have time :
First I took CiCi's famous CBH1 image and changed the colours a bit to suit my garish tastes. Then I made the background transparent, so that the wood texture would show through. Then I duplicated the image to a new layer, reduced the intensity of the second layer to zero so that it became black, then reduced the saturation to come to a more convincing grey (to emulate ambient lighting that would partially fill the shadows). Then I blurred the shadow, translated it a bit, and composited. But even a grey shadow looked wrong on the wood background (GIFs can't handle partial transparency, so it couldn't show through), so I picked up a sample colour from the background, filled the shadow with this, and then darkened it. Now it looks pretty good on its intended background (but not this one or any other, pity).
Mark Harris, Sep '98