The following are a series of guidelines we use when designing themes. These are designed to ensure our themes are as flexible and easy to use as possible.
Keep featured images in consistent proportions so that users can more easily predict thumbnail shapes & image crops.
No images should be included as part of the theme. The only images on the site should be ones uploaded by the theme user (featured images, custom header etc).
Use SVGs for site icons and any visual elements that can't be reproduced through CSS.