Probleme with parameter queue_size in image_proc node

Hello everyone, I try to change the size of the message queue in image_proc by raising the parameter ~queue_size in a roslaunch file: . . . The parameter is raising correctly, it appears in rosparm list under the name : /camera_pi/image_proc/queue_size (seem correct to me ?). But this as no effect on the queue size of the node who remaining at the default size (5). Any idea ? Thanks