Taking code from vimjay screencap to work here, this does no conversion yet.
Builds and runs but generates this error

$ roscore &
$ rosrun screengrab_ros screengrab_ros_node &
$ rosrun image_view image_view image:=/screengrab
[ERROR] [1383994501.564658075]: Unable to convert 'bgra8' image to bgr8: 'Image is wrongly formed: height * step != size  or  480 * 1920 != 1228800'
Now this node works