WP_MEMORY_LIMIT is new options introducing in WordPress 2.5 and newer. This setting increases PHP Memory for WordPress, not other applications.
Increasing memory allocate - Rachel-Bilson in Style UK
Default Setting, WordPress will trying to Increase Memory into 32M By default, WordPress will attempt to increase memory allocated to PHP to 32MB, so the best ways change the setting more than 32MB on your wp-config.php. How To Increasing memory allocate on WordPress is simple, open your wp-config.php and added this line
define('WP_MEMORY_LIMIT', '64M');
On the example above we trying to increase memory into 64MB, you can change it into 96MB or 128MB by change the parameters above. Please Notes this tricks may not works if you hosting server didn’t allow to change Memory Limit on the fly.
Source WordPress Codex, Photos Egotastic
