OptiPNG
OptiPNG performs lossless compression on PNG images; that is, it tries to reduce their file sizes using various compression algorithms and without removing any pixel data. The end result looks exactly the same as the original image.
Using it is very simple:
optipng file.pngOptiPNG spits out some information as it works, overwrites the original file with the smaller one, and shows you the reduction percentage.
We did a bunch of tests using screenshots from this very FOSSpicks section, as generated by Gimp. Occasionally we saw impressive reductions of 25–30%, but by and large the shrinkage was in the 10–15% range.
While the default options do a decent job, OptiPNG has some extra settings for choosing the optimisation level and PNG delta filters. The program’s website also has an excellent explanation of how PNG optimisation works under the hood:
