iResizer

iResizer is an image editor designed for Mac users who wish to easily resize their pictures. The program supports many popular image formats, like JPEG, PNG, TIFF or BMP, lets you choose from various resizing profiles and offers you quality results within moments.

You can import a picture in two ways: go to File -> Open or drag-and-drop the photo over the app's interface. After the image is loaded, you can use one of the marking tools from the left side of the interface to indicate the areas you wish to preserve. By doing so, you make sure that there's no quality loss in the selected segment after the resizing process is complete.

In my testing, the app worked flawlessly and offered me high-quality results.

The one thing I'd add would be the ability import multiple pictures and resize them at the same time, using a single resizing profile. This feature might prove to be of good use whenever you wish to resize a large collection of images to fit a certain style (resolution, this case).

So, you should try this program if you don't require the aforementioned feature. Otherwise, I'd suggest you look for other image resizing utilities.