PNG files are lossless compressed bitmaped images. While they are compressed, they can still attain large file sizes depending on the type of image data saved—they are best suited for images with large areas of solid colours and defined boundaries (such as logos). Unlike JPG files, they are not suited for storing photographic image data. Optimized PNG files are smaller than GIF files, and support higher-quality transparency.
Biggest savings can be achieved at a cost of reducing image quality a little:
- TinyPNG online tool that uses lossy compression to reduce the file size of PNG images
- ImageAlpha Mac tool that can reduce PNG files using pngquant, pngnq and posterization. You can often reduce size by 40-60% without noticealbe loss in quality.
- pngquant command-line tool that can reduce large 24-bit PNG to small 8-bit PNG with full alpha transparency. This is impossible to do in Photoshop!
- CompressPNG allows to reduce the pallete size to any value between 2 and 256 colors
These tools don't lose any quality, they only re-compress images and remove invisible metadata:
- ImageOptim is a Mac based only image compressor
- PNGGauntlet is a Windows PNG image compressor
- Trimage is a Linux based image compressor
- Pngcrush is a command-line application that reduces the size of PNG files losslessly. The resulting image will be identical to the original visually, but can end up being much smaller. It can be downloaded from its sourcefourge page at http://pmt.sourceforge.net/pngcrush/.
There are also online tools available that can compress PNG files. These are generally as simple as uploaded the desired file, and saving the output image.
- http://www.smushit.com/ is a service provided by Yahoo! that shrinks batches of image files
- http://www.punypng.com/ also compresses images online