Adobe PHOTOSHOP ELEMENTS 2 User Manual page 217

The JPEG format does not support animation or
transparency. When you save an image as a JPEG
file, transparent pixels are filled with the Matte
color specified in the Save For Web dialog box. To
simulate the effect of background transparency,
you can match the Matte color to the Web page
background color. If your image contains trans-
parency and you do not know the Web page
background color, or if the background is a
pattern, you should use a format that supports
transparency (GIF, PNG-8, or PNG-24).
About GIF format
The GIF format uses 8-bit color and efficiently
compresses solid areas of color while preserving
sharp details like those in line art, logos, or type.
You also use the GIF format to create an animated
image and preserve transparency in an image. GIF
is supported by most browsers.
The GIF format uses LZW compression, which is a
lossless compression method. However, because
GIF files are limited to 256 colors, optimizing an
original 24-bit image as an 8-bit GIF can subtract
colors from an image.
GIF image with Selective color, and GIF image with Web color
ADOBE PHOTOSHOP ELEMENTS 2.0
You can choose the number of colors in a GIF
image and select options to control how colors
dither in a browser. GIF supports background
transparency or background matting, in which
you blend the edges of the image with a Web page
background color. (See "Previewing and
controlling dithering" on page 216.)
About PNG-8 format
The PNG-8 format uses 8-bit color. Like the GIF
format, PNG-8 efficiently compresses areas of
solid color while preserving sharp detail like those
in line art, logos, or type.
Because PNG-8 is not supported by all browsers,
you may want to avoid this format when your
image must be accessible to a wide audience. For
more information on browser support for PNG,
see your browser's documentation.
The PNG-8 format uses a more advanced
compression schemes than GIF, so a PNG-8 file
can be 10% to 30% smaller than a GIF file of the
same image, depending on the image's color
patterns. Although PNG-8 compression is lossless,
optimizing an original 24-bit image as an 8-bit
PNG file can subtract colors from the image.
Note: With certain images, especially those with
simple patterns and few colors, GIF compression can
create a smaller file than PNG-8 compression. View
optimized images in GIF and PNG-8 format to
compare file size.
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