The file format commonly used for images on the web is JPEG (Joint Photographic Experts Group). This format is favored for its effective compression techniques that reduce file sizes while maintaining acceptable image quality, making it ideal for web usage where loading speed is crucial. JPEG supports various color depths and is widely compatible with multiple web browsers and devices. Other popular image formats include PNG and GIF, which serve different purposes, such as supporting transparency and animations. JPEG, however, remains the most prevalent choice for photographs and detailed images online.