^No, don't do that. It's called stealing.
No, it's called accessing pictures on an open, unprotected, public directory. The thumbnails are not down-scaled versions of the full prints. There are no security measures being circumvented. (The lack of a direct link isn't one.) I've looked at some of the coding for that website and there is a distinct difference between the naming of the full sized jpgs and the thumbnails. This has been the case for a long time; I remember checking this out about 2 years ago. It cannot be said that the operator of the page has no idea that the full sized pictures are available to the public. They were actively put there.
This topic has come up before and hit with the mod hammer. It likely will again, but let's be honest about what's going on here, at least. Boot these posts, keep it an open secret and let it be a "wink wink, those prints are unavailable" thing, but let's not get into accusing people of theft.