I dug into WordPress code some more and reproduced what it was doing on my own test file outside of WordPress as follows:
< ? php
/* This line below will cause cropImage to fail */
Imagick::queryFormats( 'JPG');
$img = new Imagick('leonids.jpg');
$img->writeImage('testresult16.jpg');
$img->cropImage(150,150,0,0);
$img->writeImage('testresult16-crop.jpg');
? >
Wordpress does a queryFormats call which causes cropImage to fail. Either with just a blank screen, or internal server error or a memory error if the file is large. Leaving it out will work, even on large images. ImageMagick 6.4.8 2011-02-03 Q16
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