Ecamm Live will automatically resize and crop Green Screen background images as needed. However, if you want to make a background image just the right size and shape, make it the same size and shape as your broadcast.
For example, for a 1080p broadcast, create an image that is 1920 pixels wide by 1080 pixels high.
For a 720p broadcast, create an image that is 1280 pixels wide by 720 pixels high.