We present the first tests of a digital imaging system based on a silicon pixel detector bump bonded to an integrated circuit operating in single-photon counting mode. The X-ray sensor is a 300-mum-thick silicon, 14 x 14 mm(2) sensitive area, upon which a matrix of 256 x 256 (65536) square pixel, 55 mum in side, has been built. The readout chip, named MEDIPIX2, has been developed at CERN within the MEDIPIX2 collaboration and it is composed by a matrix of 65536 readout channels exactly with the same geometry of the detector. The spatial resolution properties of the system,have been assessed by measuring the square-wave resolution function and the first images of a standard mammographic phantom were acquired using a radiographic tube in the clinical irradiation condition.