GD uses png compression.

> The optional $compression_level argument controls the amount of compression to apply to the output PNG image. Values range from 0-9, where 0 means no compression (largest files, highest quality) and 9 means maximum compression (smallest files, worst quality). A compression level of -1 uses the default compression level selected when zlib was compiled on your system, and is the same as calling png() with no argument.

See it explained GD docs.

I used png compression in my picwoodpecker -- a Tk program to choose and modify your photos (<- upvote here ;) that now is on github (<- newer version here ;)

Iirc the compression worked but in the windows machine where I'm now seems to not to have effect on the size/quality of the output (missing zlib?).

Try my (wonderful!) program above perl picwoodpecker.pl -pngcompression 0 go to advanced and set the output to png then use copy this photo from the main ineterface. Retry with -pngcompression 9

HtH

L*