CocoaSlideShow is a program for Mac that comes as an alternative solution for checking your photo collections with Preview (Mac's default image viewer). The application supports a wide variety of image formats, allows you to preview your pictures in fullscreen and slideshow modes, and helps you edit metadata info of your photos with ease.
You can work with this image utility to browse photo collections, edit image metadata, and rotate pictures. The program also enables you to create JPEG thumbnails from the input images.
Another good aspect is that you can preview your photos in slideshow mode (you can choose from the Preferences menu the way to watch the picture gallery: fullscreen or windowed mode).
When I saw the title of this program my first guess was that it's a photo slideshow creator. Unfortunately, the app only lets you check your picture collection in slideshow mode (without offering you the possibility to export the slideshow to your local folders).
As a conclusion, I can't see any harm in trying this image viewer on your Mac as it's free of charge. Still, if you need an app strictly for image browsing, I'd say you should continue working with Mac's default image viewer (Preview app).