NASA collect image data gathered by their Curiosity rovers on Mars and expose it over an API freely available to everyone specially developers, educators, and scientists. Curosity rover has its own set of photos stored in NASA's database and these photos are organized by the sol (Martian rotation or day) on which they were taken, counting up from the rover's landing date. This page shows photo taken by Curiosity based on current Earth date. Usually it is day before current day as data is not avilable through API for same day. The API results can be filtered by the camera with which it was taken. So far I guess Curosity has 7 cameras and each camera has a unique function and perspective, and they are named as follows:
Thanks to NASA & Chris Cerami for maintaining this wonderful API.