convert: missing an image filename

I am not able to get even the simplest imagemagick command to work.

Here's a simple command as seen in the Drupal log:

User notice: ImageMagick command:
/usr/local/bin/convert '/Users/pat/Sites/MITDS/site/modules/image/sample.png' -resize 400x400 -quality '75' '/Users/pat/Sites/MITDS/files/styles/home_small_logo/public/modules/image/sample.png'

Note that I am able to run this from the command line and it generates the new image correctly.

And here's the subsequent error message in the Drupal log:
User error: ImageMagick error 1: convert: no decode delegate for this image format `/Users/pat/Sites/MITDS/site/modules/image/sample.png' @ error/constitute.c/ReadImage/532. convert: missing an image filename `/Users/pat/Sites/MITDS/files/styles/home_small_logo/public/modules/image/sample.png' @ error/convert.c/ConvertImageCommand/2949. in _imagemagick_convert_exec() (line 519 of /Users/pat/Sites/MITDS/site/sites/all/modules/imagemagick/imagemagick.module).

I thought maybe there was a file permissions issue on my files directory, but chmoding it to 777 made no difference.

I tried substituting shell_exec($command), and forcing a return of TRUE from the _imagemagick_convert_exec() function in the imagemagick.module, but the convert command either fails to execute correctly in that context, or the generated file from imagemagick is subsequently deleted. I can run that command within shell_exec() from the php cli, and it works there.

I realize this may be an imagemagick module issue, but since I only see the problem when using im_raw, I thought I'd try here first. I'm also not sure if this is something I'm doing wrong (support request), or a bug. Any ideas???

I've tried a variety of module versions, including these:
im_raw 7.x-1.x-dev
imagemagick 7.x-1.0-alpha2+5-dev
imagemagick_advanced 7.x-1.0-alpha2+5-dev