There is a variety of ways to find data within the PDS.
The Planetary Image Atlas has search tools for image and image-like data from these missions:
2001 Mars Odyssey (GRS, THEMIS)
Cassini (ISS, Radar, VIMS)
Clementine (A/B-STAR, HIRES, LWIR, NIR, UVVIS)
Galileo (SSI, NIMS)
Mars Exploration Rovers (DES/HAZ/NAV/PANCAM, APXS, MB, MI, RAT)
Mars Global Surveyor (MOC, MOLA)
Viking Lander (LCS)
Viking Orbiter (VISA, VISB)
The Mars Express page at the European Space Agency has a search tool for HRSC images.
The Voyager Image Query system at the PDS Rings Node can be used to search for Voyager images.
The Planetary Photojournal is the best bet if you want to find spectacular images such as those from mission press releases.