So, something like this? (Apologies to whoever is selling this here on TB -- it was the first thread I came across with the right pics.)
Here's what I did:
1) Take pic of case with the lid shut
2) Take pic of case with the lid open and instrument inside
*NOTE* Like Dave said, make sure the camera is in the exact same spot (a tripod is a good idea), otherwise you have to mess with the perspective (like I did)
3) Open both pics in a photo editor (Photoshop is what I used)
4) Using the lasso tool or similar, trace the outline of the bass to select it
5) Copy / paste that as a separate layer in the pic of the case shut
6) Duplicate the lid shut layer and place it above the layer of the bass in the case
7) Reselect the bass-in-case layer so that it outlines the bass
8) Switch to the case shut layer (top layer), contract the selection and feather it (I used contraction: 35 pixels and feather: 12 pixels; it really depends on the size of the pic)
9) Clear the pixels from the case shut layer
Voila!
