1. A method performed by a physical computing system ( 140 ) comprising at least one processor ( 142 ) for generating a composite image ( 12 ), said method comprising: comparing ( 20 ) a background model to image data representing a set of images from a collection of images of a predetermined area using the physical computing system ( 140 ), wherein the background model is generated based on image data representing the predetermined area in the absence of traversing content; determining ( 22 ) as traversing content portions of an or each image of the set of images that differ from the background model using the physical computing system ( 140 ), based on the comparing; using ( 24 ) a target content model to determine target content in the traversing content using the physical computing system ( 140 ); and generating at least one composite image using the physical computing system ( 140 ), wherein the generating ( 26 ) comprises replacing the target content determined in the or each image of the set of images with different content at a location in the or each image that corresponds to the location of the target content.