The file format commonly used for compressing images on the web is JPEG (Joint Photographic Experts Group). JPEG is favored for its ability to significantly reduce image file sizes while maintaining acceptable quality, making it ideal for web use. This lossy compression method balances visual fidelity and file size, which is essential for faster loading times and better user experience on websites. While JPEG is widely used, other formats like PNG and GIF serve specific purposes, providing alternatives based on image content and quality requirements.