Please note that this image is copyrighted. Do not reproduce it without contacting the artist.
An area of Endeavour's floor immediately around Opportunity's shadow appears slightly brighter than the floor elsewhere, possibly because of the opposition effect, where specks of dust on the floor of the crater, seen with the Sun directly behind Opportunity's cameras, hide their own shadows from view. (This effect was first pointed out by fredk on unmannedspaceflight.com.)
Don Davis had the following to say about assembling the postcard: "The preliminary jpeg images were difficult to match due to lighting and shadow changes between exposures. I leveled and cropped and freely distorted the original mosaic, and editorially approximated the actual colors of the dusty sundial rather than using the odd colors the actual filter set uses, which is optimized for getting the subtle color differences across the scene."