NASA has several searchable online resources with public domain images.
Ames Imaging Library System (AILS)
The AILS is a library of images chronicling the projects and activities of the NASA Ames Research Center.
Digital Image Library (DIL) at the Life Sciences Data Archive
The DIL is a library of images related to life sciences space flight missions, experiments, and hardware. The DIL also includes images of ground-based experiments, astronauts, historical missions, and drawings of future missions or projects.
Earth from Space: An Astronauts View of the Home Planet
The site contains a selection of Earth images taken by astronauts in flight.
ESA Image Gallery
This site houses the main image gallery for the European Space Agency (ESA).
Human Space Flight Gallery
The site features images of NASA space flight programs.
NASA Multimedia Gallery
The gallery provides a guide to a broad array of NASA online visual resources.
Photo Gallery at the National Space Science Data Center (NSSDC)
While containing primarily planetary and astronomical images, the site includes links across government agencies to space-related images.