WinTrezur is a portable image viewer which can also store your pictures - and other files - in extremely secure archives.
The program opens much like any other image viewer. There's a folder tree on the left, a set of thumbnails on the right. Double-clicking any image displays it full-screen; pressing Esc returns you to the thumbnail viewer again, and you can view everything as a slideshow.
There are also plenty of small but useful extras here. The program can display images as raw text or binary, helping to reveal tags and other details. You're able to copy files to the clipboard, or append them, adding your images to whatever's already there. A built-in Image Properties dialog displays a vast amount of detail on the selected picture (embedded thumbnail, colours histogram, and no less than 55 individual properties covering everything from the camera make and model to the chrominance and luminance tables). And you get an image bookmarking system and an (overcomplicated) duplicate file finder thrown in.
WinTrezur's real strength comes in its encryption abilities. Select your images and they can be archived to WinTrezur's native A2R format. This may be encrypted using AES (FIPS PUB 197), Blowfish, Twofish, Serpent and more. You can combine algorithms if you like - encrypt in one, then another - using keys ranging from 128 up to 448 bits.
The password options are even more impressive. Normally you'll just enter a single password, and various indicators will show you how strong your choice might be. But there's also support for "partner passwords", where several people each have a single password, and the file only becomes accessible when they all enter what they know. And perhaps best of all, there's the option to use your images as a password. If you had 1,000 pictures in a folder, say, you might choose five favourites, in order, and that would be enough to encrypt the archive, and decrypt it later. (Just be very sure you don't change a byte of those pictures later, or you'll be locked out.)