Use 8-bit (or smaller) PNGs. Index
color PNGs will always be smaller than their 24-bit RGB counterparts.
Use JPEGs instead of RGB (24-bit)
PNGs. Photographic images are best saved in JPEG format
for use online. The resulting file sizes are smaller (with only
minimal image quality loss) and more appropriate for web delivery.
Use GIF optimization techniques.
8-bit PNGs benefit from all the same tactics used to minimize GIFs,
including limiting dithering and reducing the number of colors and
bit-depth. See Chapter 19, "GIF Format" for more information on
Avoid interlacing. Interlacing
always adds to the size of a PNG. It is usually unnecessary anyway
for small graphics or any graphic accessed via a high-bandwidth
connection or locally (as from disk or CD-ROM).
Use maximum compression (if available) for
final images. If your image tool offers control over
compression, use level 9 (or "max" or
"slowest") for the final version of your image. Use lower
compression (3 or 6) for intermediate saves. Many commercial programs
(such as Fireworks) handle compression and filter application
internally, so you may not have control over specific levels.)
Create PNGs from GIFs. Depending on
the tool you're using, you may be able to squeeze extra bytes
out of a PNG by creating it from a GIF (which is already in indexed
color format) instead of from an RGB source. If your tool offers only
a "Save as" function, this ensures that you will end up
with an 8-bit PNG. If you are starting with an RGB image, first save
it as a GIF, then open it again and save it as a PNG. This method is
unnecessary for more sophisticated web graphics tools such as
Fireworks and Photoshop/ImageReady (but Fireworks did reduce the
"sweetpea" sample image by 140 bytes when re-exported
from a GIF).
pngcrush utility. If you are serious about optimizing PNGs, you
should download Glenn Randers-Pehrson's
pngcrush application (freeware, available at
http://pmt.sourceforge.net/pngcrush/ ). It is
a command-line DOS application, but it can run in batch mode.
pngcrush takes existing PNGs and makes them
Convert transparency with the
utility. The pngquant command-line
utility (written by Greg Roelofs, one of the creators of the PNG
format) converts RGB alpha-channel transparency (32-bit) into 8-bit
palette transparency. The resulting PNG will be significantly smaller
and more suitable for web use.