A common file format used for images on the web is JPEG (Joint Photographic Experts Group). This format is widely favored due to its ability to compress images while maintaining acceptable quality, making it ideal for online use. JPEG is particularly well-suited for photographs and images with complex coloring, balancing quality and file size for faster loading times on websites. Other formats like PNG and GIF may also be used, but JPEG is predominant for photographic content.