Photoshop doesn’t present any built-in compression options when saving 24-bit transparent PNGs (PNG-24), but that doesn’t mean we can’t get the file sizes smaller.

Posterize the image first

One of the best ways I’ve found to reduce the final filesize is to posterize the image (Image > Adjustments > Posterize…) before selecting “Save for Web & Devices…” Posterizing groups similar colors together in bands and can reduce the number of actual colors in the image, thereby significantly reducing the PNG filesize. If done lightly, posterizing will not noticeably affect the image quality.

Here’s an image saved as a PNG-24 directly within Photoshop CS3. Its filesize is 272.3K.

This next image was first posterized with a setting of 16, and then saved as a PNG-24. Its filesize is 132K—less than half the filesize of the un-posterized method!

Experiment with the posterization settings

Higher amounts of posterization do not necessarily equate with smaller PNG file sizes. A setting of 15 (higher posterization) yielded a final PNG filesize of 145K for the former image, whereas a setting of 16 (lower posterization) yielded the smallest final size.

In the end, a good strategy is to increase the posterization until there is a noticeable effect on image quality, then back it off a little. At that point, experiment with the settings (+1, +0, -1, etc) to find the best balance of PNG filesize and image quality.

Smashing Magazine has an article that goes into more depth of why posterization helps, as well as containing a few other techniques to try.

Remove unneeded color correction data

pngcrush is a great utility that can further reduce PNG filesizes. Using the following command can remove unneeded color correction data, further saving us 4K of filesize.

pngcrush -rem gAMA -rem cHRM -rem iCCP -rem sRGB InputFile.png OutputFile.png

Our image is now 128K and looks like this…

If you have a Photoshop brush that won’t work, or at least stops working intermittently and you can’t figure out the problem, check to see if your Wacom graphics tablet has the Wacom mouse sitting on it. If so, take the mouse off the tablet and put it somewhere else on your desk.

Even if you’re using your regular mouse for painting, the presence of the Wacom mouse on the tablet plays havoc with the current brush’s shape dynamics setting, thus making the brush work sometimes and not others.