using the internal IM image called logo: (with colon for those special images)

size="100x100"
offset="+300+200"
opacity=50
convert logo: \
\( +clone -threshold -1 +level $opacity% -size $size xc:white 
-geometry $size$offset -composite \) \
+geometry -alpha off -compose copy_opacity -composite logo_test.png


the second line after convert creates an alpha channel. it takes a 
copy of the input image, thresholds it to completely white, then uses 
+level to change the white to 50% gray, then creates a 100x100 white 
area and places it at 300,200 over the gray background using 
composite.

the third line disables the geometry from the second line, disables 
any alpha channel already there in case and then places the second 
image as the alpha channel on the original image.

reading:

http://www.imagemagick.org/script/command-line-options.php
http://www.imagemagick.org/Usage/basics/#image_seq
http://www.imagemagick.org/Usage/compose/

and in general all of
http://www.imagemagick.org/Usage/

Fred
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