Mac OS X doesn't really come with a great utility for quick batch image conversions. You can do it in Preview, but it's not exactly a quick operation. Snap Converter makes it easy to just drop the images you want to convert, choose a format and output them wherever you want in just a few seconds.
Using it is really just as simple as dragging and dropping. You can make choices beyond the file format, but if you're satisfied with the defaults there's really nothing you have to adjust. What's also cool about Snap Converter is that on top of supporting several standard image formats and camera RAW formats, it can also convert Windows icons to Mac icons (and vice-versa). A quick image converter is never going to be some revolutionary utility, but it's very handy and a great tool to add to your app arsenal.
Snap Converter ($0.99) [Mac App Store]