Mars Images from University of Arizona HiRISE Project Tucson Citizen

Thousands of images of Mars are available from the University of Arizona’s High Resolution Imaging Science Experiment (HiRISE), see here. This catalogue contains 16,784 images. When you click on the image, you also get an explanation for the photo.
You can also view photos by Themes where the photos are grouped by process such as volcanic action, aeolian (wind), and fluvial (water) forms.