How can I change the resolution on my Mac to 720p (1280x720)?
One quick tip for finding more resolutions is to hold the Option key while clicking Scaled in System Preferences > Displays. This definitely works in OS X El Capitan and I think it worked in Yosemite at least.

For example, without Option, my 1440x900 macbook air screen gives me 1440 x 900, 1280 x 800, 1152 x 720, and 1024 x 640. But when I add Option, it adds 1140 x 810, 1280 x 720 (the one you want), and 800 x 600.