This image from the Navigation Camera (Navcam) on NASA's Curiosity Mars rover includes a bright spot near the upper left corner. The sun is in the same direction, west-northwest, above the frame. Bright spots appear in images from the rover nearly every week. Typical explanations for them are cosmic rays hitting the light detector or sunlight glinting from rocks.
The right-eye camera of the stereo Navcam recorded this frame during the afternoon of the 589th Martian day, or sol, of Curiosity's work on Mars (April 3, 2014), from the site where the rover reached a waypoint called "the Kimberley" by that sol's drive. An image taken by the Navcam's left-eye camera within one second of the same time (http://mars.jpl.nasa.gov/msl/multimedia/raw/?rawid=NLB_449790582EDR_F0310000NCAM00262M_&s=589) does not include a bright spot of this type. A pair of Navcam images in the same direction from the previous afternoon has a bright spot similarly located in the right-eye image (http://mars.jpl.nasa.gov/msl/multimedia/raw/?rawid=NRB_449700848EDR_F0301254NCAM00252M_&s=588) but not in the left-eye image (http://mars.jpl.nasa.gov/msl/multimedia/raw/?rawid=NLB_449700848EDR_F0301254NCAM00252M_&s=588).
One possible explanation for the bright spot in this image is a glint from a rock surface reflecting the sun. Another is a cosmic ray hitting the camera's light detector, a CCD (charge-coupled device). Cosmic ray patterns in Mars rover images vary from a dot to a long line depending on the angle at which the ray strikes the detector.
Photo credit: NASA/JPL-Caltech
Note: For more information, see Images From NASA Mars Rover Include Bright Spots.