It looks like stereo_image_proc links against the image_proc library but does not run_depend on it (https://github.com/ros-perception/image_pipeline/blob/hydro-devel/stereo_image_proc/package.xml). This would be the reason why in this question (http://answers.ros.org/question/70801/stereo_image_proc-error-in-hydro-error-while-loading-shared-libraries-libimage_procso/) the library is not available when installed from Debian packages.