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-tone 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 added to a file each time you save the file as a JPEG. You should always save JPEG files from the original image, not from a previously saved JPEG. An exception is the use of the source image in JPEG format from a digital camera which is usually of high quality and resolution for the Web.

You can create a progressive JPEG file. A browser immediately starts rendering a progressive JPEG when it receives the first data representing the image. As the remainder of the progressive JPEG file is received, the image increases in resolution.

The JPEG format does not support transparency. When you save an image as a JPEG, transparent pixels are filled with the Matte color. 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. The GoLive Save For Web dialog box lets you set the Matte color when optimizing a Smart Object. 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).