ImageAlpha converts 24-bit PNG to paletted 8-bit with full alpha channel. This greately reduces file sizes with only minor loss of quality. Such images are compatible with all browsers, and even degrade well in IE6.
Even images that need more than 256 colors can be made more compressible using special alpha-channel-aware posterizer.
How to use it
Drag 24-bit PNG image into main part of the window. Small images should be converted instantly. If image is large, there might be progress spinner visible in bottom right corner for few seconds.
If the quality isn't good enough, choose between Median Cut (preserves detail) and NeuQuant (best for gradients), try Dithered mode.
If the quality is great, try reducing number of colors, which will also further reduce file size.
If image color completely changes (e.g. becomes much brighter/darker), you need to remove color profile from your PNG (convert image to sRGB/gamma 2.2 in your graphics program or remove gamma with ImageOptim.)
IE6-friendly alpha makes almost-opaque colors fully opaque, to make image degrade better in IE6.
When done, save the image ⌘⇧S. ImageOptim may be able to reduce file size even more.
You can test image on different backgrounds by selecting backgrounds from the palette on the left. You can drag your own background images to that area.
“Hidden” features: background and image are draggable. Double-click toggles between 1:1 and zoomed image. Middle-click shows the original image.
ImageAlpha is mostly based on pngquant. You'll find compiled
pngquant executable in
More about PNG
My article “PNG that works” exlplains PNG8 format, and other PNG optimisation issues in more detail.