How to combine two images into one on a Mac?

Say I have two photos (or any images), for example with the following dimensions: +----------+ +----------+ | | | | | | | | |320 x 428 | and |320 x 428 | | | | | | | | | +----------+ +----------+ **I want to combine them into a single image, side by side**. So the result should be +----------------------+ | | | | | 640 x 428 | | | | | +----------------------+ **What's the simplest way to do this on a Mac?** Is it possible with some default app like Preview, or would you recommend some 3rd party tool for the job?