Add the path_to_convert option to the strategy

Seems to address this error:

Omeka has encountered an error

Omeka_File_Derivative_Exception

    ImageMagick is not properly configured: invalid directory given for the ImageMagick command!

exception 'Omeka_File_Derivative_Exception' with message 'ImageMagick is not properly configured: invalid directory given for the ImageMagick command!' in /var/www/Omeka/application/libraries/Omeka/File/Derivative/Strategy/ExternalImageMagick.php:83
Stack trace:
#0 /var/www/Omeka/application/libraries/Omeka/File/Derivative/Strategy/ExternalImageMagick.php(39): Omeka_File_Derivative_Strategy_ExternalImageMagick->_getConvertPath()
#1 /var/www/Omeka/application/libraries/Omeka/File/Derivative/Creator.php(76): Omeka_File_Derivative_Strategy_ExternalImageMagick->createImage('/var/www/Omeka/...', '/var/www/Omeka/...', 'fullsize', 800, 'image/jpeg')
#2 /var/www/Omeka/plugins/RotatePicture/controllers/IndexController.php(44): Omeka_File_Derivative_Creator->create('/var/www/Omeka/...', '1ece049b7e1bde6...', 'image/jpeg')
#3 /var/www/Omeka/application/libraries/Zend/Controller/Action.php(516): RotatePicture_IndexController->indexAction()
#4 /var/www/Omeka/application/libraries/Zend/Controller/Dispatcher/Standard.php(308): Zend_Controller_Action->dispatch('indexAction')
#5 /var/www/Omeka/application/libraries/Zend/Controller/Front.php(954): Zend_Controller_Dispatcher_Standard->dispatch(Object(Zend_Controller_Request_Http), Object(Zend_Controller_Response_Http))
#6 /var/www/Omeka/application/libraries/Zend/Application/Bootstrap/Bootstrap.php(101): Zend_Controller_Front->dispatch()
#7 /var/www/Omeka/application/libraries/Zend/Application.php(366): Zend_Application_Bootstrap_Bootstrap->run()
#8 /var/www/Omeka/application/libraries/Omeka/Application.php(79): Zend_Application->run()
#9 /var/www/Omeka/admin/index.php(28): Omeka_Application->run()
#10 {main}