1. An apparatus ( 130 ; 230 ) for storing digital images ( 300 ), comprising a storage device ( 134, 136 ; 234, 236 ) capable of storing a current reference image ( 310 ) and a plurality of differential images ( 320 ), and an image processing device ( 132, 244 ) coupled to the storage device, wherein the image processing device is adapted to receive an individual digital image ( 300 ), determine a difference between the individual image and the current reference image ( 310 ), and store this difference as a differential image ( 320 ) in the storage device, characterized by: reference image updating means ( 132 ; 232, 244 ) for determining when to generate a new reference image ( 310 ), for generating the new reference image, for replacing the current reference image with the new reference image, and for storing the new reference image as current reference image in the storage device ( 134, 136 ; 234, 236 ).