Images should be at least 1200 pixels wide (formerly 696 pixels wide). Every page must contain at least one image (whether or not you include markup). Google will pick the best image to display in Search results based on the aspect ratio and resolution.
Google has updated its article schema document for AMP articles to require larger images in your markup. Previously, the minimum requirement for your image sizes was 696 pixels wide and 300,000 pixels in total, but now it is 1,200 pixels wide and 800,000 pixels in total.
Old Amp requirements from the Articles schema developer document: