About JPEG format

The JPEG format supports 24-bit color and preserves the broad range and subtle variations in brightness and hue found in photographs and other continuous-toned images. JPEG is supported by most browsers.

JPEG compresses file size by selectively discarding data. Because it discards data, JPEG compression is referred to as lossy. A higher quality setting results in less data being discarded, but the JPEG compression method can degrade sharp detail in an image, particularly in images containing type or vector art.

Note: Artifacts, such as wave-like patterns or blocky areas of banding, are created each time you save an image in JPEG format. These artifacts accumulate each time you resave the image as the same JPEG file; therefore, you should always save JPEG files from the original image, not from a previously saved JPEG.

You can create a progressive JPEG file, in which a low-resolution version of the image appears in a browser while the full image is downloading.

The JPEG format does not support transparency. When you save an image as a JPEG file, transparent pixels are filled with the Matte color, as specified in the Optimize palette. If you know the background color of the Web page where you will place the image, you can match the Matte color to the Web page background color to simulate the effect of background transparency. If your image contains transparency and you do not know the Web page background color, or if the background will be a pattern, you should use a format that supports transparency (GIF, PNG-8, or PNG-24).

Optimizing Images for the Web > Choosing a file format for optimization > About JPEG format