The file format commonly used for compressing images on the web is JPEG (Joint Photographic Experts Group). JPEG files offer a good balance between image quality and file size, making them ideal for use on websites and online applications. This lossy compression method reduces image data by discarding some information, allowing for quicker loading times and more efficient storage. JPEG is widely supported across various platforms and devices, making it the standard choice for web images, especially photographs and complex graphics.