Guide background images are a critical part of making your guide look great. We give you the ability to define both a desktop background image and a mobile background image. Reason being is desktop (or laptop) background images should be landscape (wider than tall) and mobile background images should be portrait (taller than wide). As for optimal sizes, there is not a perfect size but something along these lines is recommended:
- Desktop: 1900px wide X 1200px tall
- Mobile: 550px wide X 1000px tall
We really like this free online tool for resizing images.