How to reduce PNG file size for web?

RIOT can do that, and a lot more.

Radical Image Optimization Tool (RIOT for short) is a free image optimizer that will let you to visually adjust compression parameters while keeping minimum file size.

It uses with a side by side (dual view) or single view interface to compare the original with the optimized image in real time and instantly see the resulting file size.

The image optimizer is lightweight, fast and simple to use, yet powerful for advanced users. You will be able to control compression, number of colours, meta data settings and much more, and select image format (JPEG, GIF or PNG) for your output file.

Another possibility is to use a combination of pngquant, pngout, and pngcrush, as described here, but this is from the command line.

Here are the instructions how to convert png24 images into png8 for ie6 goodness, all through the command line using open source tools (i think) pngquant +pngout + pngcrush.

1- quantize image into 256 (so basically png8′s look crap with large sprites or sprites with a large colour range).

pngquant 256 some_24_bit.png

2- convert image form a png24 to a png8

pngout -c3 -d8 -y -force some_24_bit.png some_8_bit.png

3- compress image

pngcrush some_8_bit.png -bit_depth 8 -brute -rem alla -reduce some_8_bit_small.png

PNG is a lossless compression format -- without changing the source material in the file, there's very little way to increase compression beyond the default highest level compression. The only real way to reduce the size will be to change the image, either by reducing resolution, or colour depth so that the PNG compression has less complexity to work with.

If you want high levels of compression for the image and don't care about lossless data storage, use JPEG.

Try TinyPNG

From their website:

How does it work?

When you upload a PNG (Portable Network Graphics) file, similar colors in your image are combined. This technique is called “quantization”. Because the number of colors is reduced, 24-bit PNG files can be converted to much smaller 8-bit indexed color images. All unnecessary metadata is stripped too. The result: tiny PNG files with 100% support for transparency.

They have two options:

• Drag and drop the images onto their website and it gets automatically processed. Free service. (Up to 20 images. Max 5 MB each)
• Photoshop plugin. This costs $.

I have not tried the PS plugin so can't comment on how well it works.