1. A method of monitoring output data from a video camera, comprising the steps of: analyzing input data to produce a model of a background image; comparing incoming image data against said model of a background image to determine pixels within incoming images that define a foreground object; for each incoming image, deriving a level of activity by measuring the proportion of pixels in the image that belong to a foreground object; comparing the level of activity of each incoming image with a threshold value to identify images having a high level of activity created by the introduction of a foreground object; identifying periods of activity, each said period of activity being composed of a plurality of consecutive images having a high level of activity; and generating an exemplar image from each said period of activity, such that said step of generating said exemplar image comprises for each period of activity (i) selecting an exemplar image from a peak level of activity occurring within the period of activity; (ii) selecting an exemplar image from substantially the center of the period of activity, or (iii) combining a plurality of images from the period of activity.