Create MacOS style screenshots with drop shadow using Imagemagick
The following appears to work. I've done it twice -- once to show the shadow more explicitly, and then once to show the desired, finished output.

# adds the shadow
convert source.png \( +clone -background black -shadow 100x40+0+16 \) \
+swap -background none -layers merge +repage output.png

# adds the additional space like MacOSX
convert output.png -bordercolor none -border 32 target.png




Caveats:


The padding on should be 20px less on the top and 20px more on the bottom; this solution is not perfect in that regard.
The OSX screenshow draws a white border around the image, including the rounded corners. I could not duplicate that.
The shadow from osx screencapture looks more blurred but I don't know how to measure the blue and calibrate the solution to that.


Highlighted shadow to show shape and blurring