I have a png photo, sluggo.png (354 pixels by 502 pixels, 72 dpi). Using sluggo.png, I created a layer in Photoshop and the drew an ellipse, feathering it 30 pixels. I inverted the ellipse, filled the inverted area (exterior of ellipse to the edges of the image) with black, and then exported the image to mask.png -- having the same dimensions and dots per inch as the image it was derived from: sluggo.png.
Goal: create an elliptically cropped image with transparency outside of the ellipse.
Here's the command I used based upon my study of the above link to the section titled "Difference Image Masking and Feathering". It is practically identical to the examples, yet I must be missing something very obvious as it did not produce the desired result:
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convert sluggo.png mask.png +matte -compose CopyOpacity -composite output.png
the output.png was blank. I was expecting an oval like framing of sluggo. "Oh No!!!!"
sluggo.png

mask.png

output.png (blank white)

Using ImageMagick for Windows on XP Pro SP 3:
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Version: ImageMagick 6.6.4-2 2010-09-14 Q16 http://www.imagemagick.org
Copyright: Copyright (C) 1999-2010 ImageMagick Studio LLC
Features: OpenMP