There is indeed a few NASA sites where you can grab the original .tiff files; I've got a 130mb .tiff for example I grabbed; it was taken from that most recent probe that arrived at Mars, can see Opportunity parked next to a crater in it.. can even make out its shadow! From orbit.
Each little agency within NASA hordes its goodies in different places on occasion, which makes it difficult, but..
A lot can be dug up through this:
http://photojournal.jpl.nasa.gov/index.html
That first nebula in the slide show is in there; 45mb full-res.
Other sites, like this one, can let you dig up more peculiar (though usually less interesting) pictures.. Like whatever happens to be in Spirits rear-view mirror at the moment.
http://marsrovers.nasa.gov/home/
Hubble's is better than the rovers, IMHO, 'cause you can get both a simple explination plus a much more technical summary of what the picture is in some of them.
http://hubblesite.org/
Here is Spitzers
http://www.spitzer.caltech.edu/Media/me ... ndex.shtml
I had links to those, and more, except for Spitzer, but I found Spitzer super-easy with Google. Some of these images wouldn't fit on a 30" Dell widescreen without resizing, so hopefully they fill you guys up.
Happy astro-porn.
Edit: Typical government, lol, I found that 'Eye of God' picture four different places, and each claimed that their 'highest resolution' copy was the highest quality version available -- yet all four had significantly different file sizes, ranging from 11 to 45mb. If you really want the highest-quality version available (like me, I tend to make desktop backgrounds out of them), looking around might net better results.