A user terminal for participating in video calls comprises: an encoder having a frame size, being the size in pixels at which it encodes frames of video; and a pre-processing stage which supplies a sequence of frames to the encoder at that frame size, each frame comprising at least an image region representing a source video image at a respective moment in time. The pre-processing stage is configured to supply at least some of the frames to the encoder in a modified form, by resizing the source video image to produce the image region of each modified frame with a size smaller than the frame size of the encoder, and combining with a border region such that the modified frame matches the frame size of the encoder. The encoder encodes the frames at the frame size and transmits them to a receiving terminal as part of a live video call.