Recently, I have found an excellent tool for optimizing PNG for use in web design and I'd like to share it because I think it's really great - you can do things that Photoshop users can only dream about. I'm talking about [8-bit PNG with alpha transparency that Photoshop cannot save and so far searching the web mostly revealed that you can either [url=http://www.sitepoint.com/png8-the-clear-winner/]use Adobe Fireworks for this purpose or some command-line tools. But for a few weeks now I've been using [url=http://x128.ho.ua/color-quantizer.html]Color Quantizer - a small Windows program that can optimize PNG's in a convenient interface. The page is in Ukrainian so you need to click "Скачать" to download (warning: I don't want anyone to run into problems since [url=https://www.virustotal.com/]VirusTotal](http://calendar.perfplanet.com/2010/png-that-works/) reports some 2 suspicions on the exe so please use it at your own risk! I treat them as false positives but not everyone wants to think like me. The downloads at Softpedia and Softoxi lead to older versions).
The program lacks documentation but is pretty easy to use. I basically load a PNG-24 image from Photoshop Elements or GIMP and in most cases I save it as a 256-colour PNG in Color Quantizer and the file size savings are usually between 2 and 4 times with imperceptible quality loss, which I find remarkable. Interestingly, I can reduce the colours to a greater number than 256, like 512 or 1024, and then the resulting image is 24-bit but still the file size reduction is pretty good because fewer colours compress much better. Of course, smooth transparency gradients are always preserved (with some rare exceptions). Some of the settings are a bit cryptic but with some experimentation it's easy to grasp them.
The program (newest version) can also batch-process images so whenever I finish a new layout I feed all the PNG's into Color Quantizer and get very good compression ratios.
For people using Macs there's Image Alpha which supposedly has similar features but I can't comment on it because I don't work on a Mac.
And the easiest method of all is the online service TinyPNG where you can get your PNG's optimized automatically into 8-bit PNG's with alpha - for me it works well in 80% of cases but sometimes I get some false or phantom colours and then I have to use Color Quantizer.
I think in today's web design where all browsers support alpha transparency the need for good PNG compressors is much higher than before. A few physically small 24-bit PNG images straight from Photoshop can amount to a lot of kilobytes and slow down page loading time a lot. It's a shame that popular graphic software is still inadequate for the job and we have to be searching for such exotic solutions but at least they begin to appear.
If someone knows of some other tools for PNG compression it would be great to hear as well. What I mentioned above is the best I could find so far (for free).