Imagick can handle those formats if your server has it available. (requires PHP5 and Zenphoto 1.3.0)
If your server doesn't, then you could create a plugin similar to the `class-textobject` plugin that provides functionality for formats that GD can't read. It wouldn't even need to display the image; it could, for instance, simply display a message informing the user that the file can't be displayed and perhaps provide a link to download the file.