All of the images presented on NSSDC's Image Catalog are in the public domain. As such, they may be used for any purpose. NSSDC does ask, however, that you acknowledge NSSDC as the supplier of the data. In addition, where the source of the image (by project or as a specific person) is credited in the text, you should also acknowledge that, too.
There are a number of reasons you might not have found what you're looking for in the NSSDC Image Catalog. First, the image(s) may not yet be available. Second, the image(s) may be available, but not (at least yet) from NSSDC. Third, it may be something which no one has suggested be put up. Lastly, it may be outside the province of NSSDC's responsibilities. You might also try first looking through the list of other sites. The list is not a comprehensive listing of all sources of images, but covers most of the areas we are asked about most frequently.