ViewIt

ViewIt is more than just an image preview utility designed for Mac. This program also enables you to find duplicated pictures, convert photos to other formats, and batch rename images within moments.
ViewIt comes as an alternative to using Mac's Preview tool for opening images on your computer. Unlike Mac's default utility, this tool enables you to browse your images by using your keyboard keys.

The tool supports a wide variety of input formats and can even import pictures from a digital camera that's being connected to your Mac. You can choose between various display modes, sort the imported photos if needed, and toggle fullscreen mode with just one click.

There are a couple of things I don't like about this utility. For starters, the built-in image converter supports a limited amount of output formats (even though ViewIt supports dozens of input formats): JPEG, GIF, PNG, and TIFF. Another thing I don't like is the appearance of the interface which looks like the main window of a utility that was created a long time ago. Last but not least, you should also take into consideration the fact that you can find cheaper solutions for previewing, editing, and even converting images on your Mac.

So, it's up to you to decide if it's worth buying ViewIt for your Mac or not.