To ensure both speed and quality on mobile devices in Shopify, use the shopify recommended product image size of around 2048 x 2048 pixels for high resolution. Shopify automatically adjusts the size of images based on the device, ensuring fast load times while preserving quality. For further optimization, consider using apps like Image Optimizer or TinyIMG, which automatically compress images and resize them for mobile devices without manual intervention. These tools help maintain quality while improving site speed on both mobile and desktop platforms.