Chris, I think you hit it right in the explanatory posting of you found on the web.  I looked into this sometime ago.  (actually you remember the images I made of you in a space suit on the surface of Mars as a spoof? Those were NASA astronaut photos) 

My understanding was exactly as you said.  The images a pretty much for public consumption. NASA and JPL doesn't want someone using their logos or the images as a misrepresentation of them endorsing user stuff.  That was what I read then.