A method and an apparatus for transmitting an image in which the image is sampled at a first plurality of points. The samples are then subjected to a two-dimensional unitary transform to produce a second plurality of transform coefficients, the second plurality being smaller than the first plurality and being obtained either by selection or truncation of the transform to provide the coefficients containing only the lower frequency components of the image. The transform coefficients are transmitted and on receipt converted by the inverse transform to a second plurality of image samples. Suitable transforms include the Hadamard and the discrete cosine. The image transmission is described as part of a viewdata system.