With gradients you can add a sense of depth or shading to your photography. Learn how to add gradients using the Gradient tool in Pixelmator. This is useful for both photographic and design tasks.

https://www.youtube.com/watch?v=fdwrd4m_deg

This tutorial is a single movie from the Up and Running with Pixelmator course by lynda.com author Rich Harrington. The complete course is 3 hours and 33 minutes and shows how to get up and running with Pixelmator, a full-featured image-editing tool for the Mac.