Today's Image of Mars is a closeup of a mysterious bright object located at Rocknest on Mars taken by MSL Curiosity's Mars Hand Lens Imager (MAHLI). The image shows an area spanning only 4cm across, which should give you a better idea of how big these bright particles are. MSL took this image after it had taken a scoop of the terrain.
The MSL team was originally worried that the bright objects could be a part of the rover that had fallen off, as this has already happened once before, but after further image analysis it was determined that these bright particles are natural to Mars.