Today's Mars Photo was taken by by Viking I Orbiter as Phobos' shadow passed over the surface of Mars. Viking I Lander took a simultaneous picture, which was used in conjunction with the Orbiter's image identify the position of the shadow, and thus the lander itself. The shadow of Phobos is in the center of the image and is about 60 by 120km across and in the Chryse Planitia region just a few kilometers north of the Viking I Lander. Maurnee Vallis is shown in the below and to the left of the center shadow.
This image is larger than the original so it will link to the Wikipedia Page called Transit of Phobos from Mars