On the night of 3 July 2003, the Mars Express spacecraft was pointed backwards to obtain a view of the Earth-Moon system from a distance of 8 million kilometres while on its way to Mars. This image is the first picture of planetary objects obtained by the Mars Express High Resolution Stereo Camera (HRSC). Although the spatial resolution is low at this great distance, the picture gives a good indication of what to expect from Mars Express in its orbit around Mars. At only 250-300 kilometres above Mars, the camera will obtain very high-resolution images, in brilliant colour and impressive 3D of most of the Martian surface, at resolutions of up to 2 metres. The image was built by combining a super resolution black-and-white snap-shot image of the Earth and the Moon taken by the HRSC with colour information obtained by the blue, green, and red sensors of the instrument.
Image Credit: ESA/DLR/Freie Universität Berlin
Explanation from: http://mars.jpl.nasa.gov/express/newsroom/pressreleases/20030717a_image01.html