image_transport provides transparent transport of images in compressed formats. This transparency means that on the subscriber side in the callback, you receive a sensor_msgs::Image regardless of which transport has actually been used. That's a feature, because this way, the transport can be transparently exchanged, without any need for code modifications on the consumer side.
void savingImagesCb(const sensor_msgs::ImageConstPtr& msg)

If you actually want to receive the compressed image, using a plain subscriber on the compressed topic directly should work (e.g. subscriber to /camera/image_raw/compressed for instance).