exec() vs shell_exec()

excerpt from php manual:
exec  Execute an external program http://www.php.net/manual/en/function.exec.php
shell_exec  Execute command via shell and return the complete output as a string http://www.php.net/manual/en/function.shell-exec.php
So maybe you can explain what exactly you need to achieve so we can advice the way you should proceed.
Cheers

All I need to do is resize an image using ImageMagick.

convert -size 240x180 image.jpg -thumbnail 120x90 thumb.gif

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