A video telephony system comprising a transmitting user terminal and a receiving user terminal for participating in video calls; wherein the transmitting user terminal comprises: - an encoder operable to switch between different ones of a set of predetermined frame sizes, each frame size being a size in pixels at which the encoder encodes frames of video, wherein the frame size of the encoder remains constant at a first frame size over a consecutive plurality of said frames; and - a pre-processing stage configured to supply a sequence of frames to the encoder at said frame size, each frame comprising at least an image region representing a source video image at a respective moment in time; and wherein the receiving user terminal comprises: - a decoder configured to receive and decode the sequence of frames from the encoder; and - a renderer configured to render the image region of each decoded frame; wherein the encoder is arranged to encode each of the plurality of frames at said first frame size; and wherein the pre-processing stage is configured to dynamically adapt the size of the image region in dependence on one or more of: the channel conditions affecting said transmission, the processing resources of said transmitting user terminal, the processing resources of the receiving terminal; the pre-processing stage is configured to perform said adaption by supplying at least some of said plurality of frames to the encoder in a modified form; and the pre-processing stage is configured to perform said adaptation by resizing the source video image to produce the image region of each modified frame with a size smaller than the first frame size of the encoder, and combining with a border region such that the modified frame matches the frame size of the encoder, characterized in that: the pre-processing stage is configured to perform the adaptation of the size of the image region while the frame size of the encoder remains constant at the first frame size over said consecutive plurality of frames, the pre-processing stage being configured to perform said adaptation of the size of the image region in gradated steps between the predetermined frame sizes, being relatively finely gradated relative to steps in the predetermined frame sizes between which the encoder is configured to switch; the resizing comprises cropping a portion of the source video image and replacing the cropped portion of the source video image with the border region; the encoder is arranged to transmit the encoded frames to the receiving terminal as part of a live video call; the transmitting user terminal is also configured to signal information on said resizing to the receiving terminal, for use in scaling up the image region of each of the modified frames for display by the receiving terminal; and the renderer comprises a resizing stage configured to receive the information on the resizing, and based on said information to scale up the image portion of each of the modified frames to said first frame size, discarding the border region, the renderer being configured to render the image region of each of the decoded modified frames at said first frame size. - an encoder operable to switch between different ones of a set of predetermined frame sizes, each frame size being a size in pixels at which the encoder encodes frames of video, wherein the frame size of the encoder remains constant at a first frame size over a consecutive plurality of said frames; and - a pre-processing stage configured to supply a sequence of frames to the encoder at said frame size, each frame comprising at least an image region representing a source video image at a respective moment in time; and - a decoder configured to receive and decode the sequence of frames from the encoder; and - a renderer configured to render the image region of each decoded frame; the pre-processing stage is configured to perform the adaptation of the size of the image region while the frame size of the encoder remains constant at the first frame size over said consecutive plurality of frames, the pre-processing stage being configured to perform said adaptation of the size of the image region in gradated steps between the predetermined frame sizes, being relatively finely gradated relative to steps in the predetermined frame sizes between which the encoder is configured to switch; the resizing comprises cropping a portion of the source video image and replacing the cropped portion of the source video image with the border region; the encoder is arranged to transmit the encoded frames to the receiving terminal as part of a live video call; the transmitting user terminal is also configured to signal information on said resizing to the receiving terminal, for use in scaling up the image region of each of the modified frames for display by the receiving terminal; and the renderer comprises a resizing stage configured to receive the information on the resizing, and based on said information to scale up the image portion of each of the modified frames to said first frame size, discarding the border region, the renderer being configured to render the image region of each of the decoded modified frames at said first frame size.