Transmission of image date during communication sessions

A method and system of controlling a communication session between a first mobile device (1A) and a second mobile device (1B), each of which is associated with a communications network (7A,7B) is described. Image data associated with the first device (1A) is stored on image server (19A). A communication link is initiated between the first device (1A) and the second device (1B) for transmitting speech data from the first device (1A) to the second device (1B), and including transmitting a message to the second device (1B) for instructing that device to display the image data. The image data is displayed on the second device (1B) in response to the message contemporaneously with reproducing the speech data. The image data stored in the image server (19A) of the first network (7A) associated with the first mobile terminal (1A) is transmitted to the image server (19B) associated with the second network (7B) (by data exchange between respective IMS cores 11A and 11B). PoC server (13B) controls the sending of this image data to mobile terminals (1B) associated with the second network (7B). Once received by mobile terminal (1B), the image data is stored thereon, and is automatically reproduced each time speech data is transmitted to device (1B), thereby obviating the requirement to re-transmit this image data to the device (1B). The embodiment described relates to transmitting image data automatically during a push-to-talk over cellular (PoC) communication session.