So far they've released just shy of 5000 high quality public domain images, with an intent for this to grow. You can read more about this on the The Getty Iris, or you can jump right to the Getty search portal (I've taken the liberty of specifying the Open Content Images for your perusal). Links below.
Getty Iris posting: http://blogs.getty.edu/iris/open-content-an-idea-whose-time-has-come/
Getty Image Search (Open Content): http://search.getty.edu/gateway/search?q=&cat=highlight&f=%22Open+Content+Images%22&rows=10&srt=a&dir=s&pg=1