There are a few central archives online where most planetary imaging data can be found, but each mission may have its own specific resources elsewhere on the Web.
NASA's spacecraft data belongs to the public and has always been available. Since 1966, when the National Space Science Data Center (NSSDC) was established at NASA's Goddard Space Flight Center, the archived data from all science missions has been made available to researchers upon request and for a nominal fee. All NASA missions are required to archive their data with the NSSDC after a proprietary period has elapsed. Similar archives exist for the Japanese and European space agencies.
In 1977, NASA established a system of Regional Planetary Imaging Facilities (RPIFs). RPIFs are like NASA public libraries, storing collections of image data in both digital and print formats. Until a few years ago, these resources were mostly used only by professional researchers. As early as the 1980s, the NSSDC was distributing some data electronically, but few in the wider public had the bandwidth, hardware, software, or training necessary to download, process, and interpret the data.
Things have changed for the better. Data from NASA and many other agencies' missions are now easily accessible through search tools provided by the eight distributed Nodes of NASA's Planetary Data System, or PDS. The PDS is responsible for archiving the data from the planetary missions. The PDS is actually a distributed system, with a Central Node and sub-nodes located at various institutions around the country. The Central Node of the PDS is located at the Jet Propulsion Laboratory in Pasadena, California. The PDS sub-nodes are organized by scientific discipline: Planetary Rings, Atmospheres, Geosciences, Planetary Plasma Interactions, Imaging, Small Bodies, Navigation, Radio Science, and Engineering. NASA planetary missions are required to deliver data to the PDS within a time period that varies from mission to mission but is usually six months to a year.
The National Space Science Data Center, or NSSDC, is another NASA archival storage center for data obtained from NASA space missions, located at the Goddard Space Flight Center in Greenbelt, Maryland. The NSSDC serves as NASA’s central data repository for all NASA missions. The NSSDC archives contain data products provided to them by the PDS and data products from missions not supported by the PDS. The NSSDC does not support discipline nodes analogous to the PDS structure. Planetary image data is available for purchase on CD-ROM; it is mostly not available online. However, the NSSDC provides detailed background information on missions and instruments (including non-NASA missions) useful to interpreting the data. To find this background information, follow each mission's link to "Experiment Information" and "Data Set Information."
The European Space Agency's online archive of planetary imaging data is the Planetary Science Archive, or PSA. It is similar to the PDS, though data is delivered to it more slowly. (NOTE: Much of their data is made available by FTP. For some reason, it does not work to access their FTP site through Chrome, but other browsers/clients do work.)
Resources for Specific Missions
This section is very far from complete.
Camera: "ISS" (Imaging Science Subsystem), two cameras, framing CCDs, 1024 pixels square. "WAC" (Wide Angle Camera): field of view 61 mrad, angular resolution 60 μrad, two overlapping 9-position filter wheels. "NAC" (Narrow Angle Camera): field of view 6.1 mrad, angular resolution 6 μrad, two overlapping 12-position filter wheels.
Data:The PDS Imaging Node has complete data volumes in PDS format. Volumes can be searched through Planetary Image Atlas. The PDS Rings Node provides the volumes online also, and, helpfully, entire volumes zipped together. These data are delivered to the PDS within nine to twelve months of their acquisition. The Planetary Society provides selected images of Cassini's icy moons, converted to PNG format.
Data:The Hayabusa Project Science Archive has complete science data set (which does not yet include images taken for navigational purposes) in FITS format. The Planetary Society provides the same data set converted to PNG format [NOTE: this resource has not yet been migrated to the new site. If you miss it, send us an email and we'll prioritize it.]
Mariner 9 (Mariner '71)
Camera: Vidicon, 2 cameras, 832 samples x 700 lines. Wide-angle camera: field of view 15.5 x 10.5 degrees. Narrow-angle camera: field of view 1.41 x 1.06 degrees.
Metadata:Spreadsheet (Microsoft Excel format, zipped, 7.5 MB) containing an index to all the images.
Camera: Vidicon, 832 samples x 700 lines, 8-position filter wheel
Data: 12,000 images in total. PDS Imaging Node has complete data set in VICAR format (CODMAC Level 0). Read images with Piotr Masek's Mariner View. The Mercury Mariner 10 Image Project has the Mercury portion of the data set in an online searchable database. Images can be downloaded in PNG, ISIS (.cub), or PDS (.img) formats, in either "raw" or "clean" versions. Clean versions appear to be geometrically corrected, with reseaux removed.
Camera: "HRSC" (High Resolution Stereo Camera), pushbroom CCD, line width 5184 pixels, 9 channels pointing from 15 degrees ahead to 15 degrees behind Mars Express' orbit. One "stereo," photometric, and infrared channel point behind; "nadir," green, and blue are nadir; and "stereo," panchromatic, and red point forward. "SRC" (Super resolution camera), framing CCD, 1024 pixels square; SRC pixels are 23% as wide as HRSC pixels. Because of the varying altitude of Mars Express, HRSC resolution varies from a maximum of 10 meters per pixel. More information here.
Data: Available from the HRSCView interface at Freie Universität Berlin, which allows very easy browsing of data, including overlay of image data on derived digital terrain models, and then downloading of actual data in PDS or VICAR formats via FTP.
Camera: "VIS" (Visual Imaging Subsystem), two identical vidicons on each spacecraft, FOV 1.51 by 1.69 degrees, 1204 samples x 1056 lines. Six-position filter wheels contain blue, "minus blue," violet, green, red, and clear filters. More information here.