PNG compression works differently from JPEG. Instead of removing visual detail, it reduces the number of colors in your image using advanced quantization algorithms. This can dramatically shrink file sizes—often by 70% or more—while preserving the crisp edges and transparency that make PNG ideal for graphics.
PNG images are often RAW/Source material, generally the compression of these files attempts to be lossless compression, whereby the image will look the same but with less color information (PNG-24 format supports over 16 million colors).
On BlueHost it often errored out when attempting to compress a 700k PNG file with GD, utilizing the available ImageMagick binaries on the system allowed compression processing without issues. From our tests, there isn't much to really compress since PNG is a lossless format already.
