Save images as 8-bit grayscale in image_view extract from rosbag file

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Hello,

I use a launch-file in order to extract images from a stereo-camera (stereo/left & stereo/right). Thereby I use the package "image_view" with the node "extract" to grab the images from my bagfile.

<launch>
   <arg name="path" default="/home/viki/bagfiles" />
   <arg name="bagfile" default="bagfile_stereo_raw.bag" />
   <arg name="folder_name" default="default_image_folder" />
   <arg name="format" default="png" />

   <param name="/use_sim_time" value="true" />

   <node name="rosbag" pkg="rosbag" type="play" 
      args="--clock -d 4 $(arg path)/$(arg bagfile)" />


   <node name="extract1" pkg="image_view"   type="extract_images" respawn="false" output="screen"                 cwd="ROS_HOME" >
   <remap from="image" to="/stereo/left/image_rect" />
   <param name="filename_format" value="/stereo_images/$(arg folder_name)/left/left_image%04i.$(arg                  format)"/>
   </node>

   <node name="extract2" pkg="image_view" type="extract_images"  respawn="false" output="screen"                  cwd="ROS_HOME" >
   <remap from="image" to="/stereo/left/image_rect" />
   <param name="filename_format" value="/stereo_images/$(arg folder_name)/right/right_image%04i.$(arg                  format)"/>
   </node>

</launch

I now receive all the desired images but they are 24-bit images. Now my question is: How can I setup the launch-file in order to receive 8-bit (grayscale) images? I cannot find any documentation regarding this. Also my question is how to obtain parameters from nodes like "extract" in ROS in general. I found out that there is a parameter called "file_format" for "extract" in the ROS answer sites but how can I see all the different parameters there are for this node. (I am sure my question would resolve by acquiring this info).

Also one other question: I need to create the directories before saving the images (with the launch file) into them. Is there a way that the roslaunch file creates the desired directories by itself?

Thanks in advance! Hector