How to Synchronize two topics ?

edit

Hi I am new to ROS, I am trying to synchronize two subscribed topics from last few days. I have tried almost all solutions mentioned in this forum but still struggling to get it synchronized. The callback function is never called. Below is my code as well CMakeLists.txt. Can anyone plz suggest where I am going wrong or what changes I should make.

Here is my code:

#include <message_filters/subscriber.h>
#include <message_filters/time_synchronizer.h>
#include <message_filters/synchronizer.h>
#include <message_filters/sync_policies/approximate_time.h>
#include <sensor_msgs/Image.h>
#include <boost/bind.hpp>
using namespace sensor_msgs;
using namespace message_filters;

void callback(const ImageConstPtr& image1, const ImageConstPtr& image2)
{
  ROS_INFO_STREAM("Hi I am in callback");
}

int main(int argc, char** argv)
{
  ros::init(argc, argv, "test_messge_filter");
  ros::NodeHandle nh;
  message_filters::Subscriber<Image> image1_sub(nh, "image1", 1);
  message_filters::Subscriber<Image> image2_sub(nh, "image2", 1);
  typedef sync_policies::ApproximateTime<Image, Image> MySyncPolicy;
// ApproximateTime takes a queue size as its constructor argument, hence MySyncPolicy(10)
  Synchronizer<MySyncPolicy> sync(MySyncPolicy(10), image1_sub, image2_sub);
  sync.registerCallback(boost::bind(&callback, _1, _2));
  ros::spin();
  return 0;
}


CMakeLists.txt:
find_package(catkin REQUIRED COMPONENTS
pcl_ros  
roscpp
sensor_msgs 
image_transport  
message_filters  
std_msgs
message_generation 
cv_bridge
)
find_package(OpenCV REQUIRED)
find_package(Boost REQUIRED COMPONENTS 
signals
) 

catkin_package()
include_directories(
${catkin_INCLUDE_DIRS} 
${OpenCV_INCLUDE_DIRS} 
${Boost_INCLUDE_DIRS}
)

add_executable(pub_sub_image_pointcloud src/pub_sub_image_pointcloud.cpp)
target_link_libraries(pub_sub_image_pointcloud

${catkin_LIBRARIES}  
${OpenCV_LIBRARIES}
${Boost_LIBRARIES}
 )