Browsing, I saw lovely photographs, objects, paintings--a wide range of items that have been imaged. When I went to the search page, however, and entered this search in the caption search box: "cat head"
my search retrieved one image, a portrait of Captain Sir Charles Hamilton. Apparently, this text is responsible for responding to my phrase search:
This half-length portrait in kit-cat format shows the sitter with his head turned to the right.
Since the FAQ doesn't include advice on constructing search terms, I'm inferring that they don't offer phrase searching. I then tried various terms and variations. I was hoping to find images that I had found through their website before, e.g., "PAD8643 Gaff, Cat Head, Driver Boom, Bow Sprit," a watercolor--but I didn't find it on nmmimages.
It is a great website. On a fast connection, it's easy to navigate, and if you see something you like, there is a link at the bottom directly to their print sales. But I'm not sure how useful the site is for the serious researcher--there are clearly more images available on the main NMM website, but how many more? We don't know. And images that are on the NMM Flickr site are also not appearing on nmmimages (when I looked).
The casual user, or pleasure browser, might enjoy nmmimages, but the casual user might discover more NMM images through a Google Images search by including the phrase "national maritime museum"--I know I did.