imagemagick resizing and quality PNG

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  1. You can further reduce quality of PNG by using posterization:

    https://github.com/pornel/mediancut-posterizer (Mac GUI)

    This is a lossy operation that allows zlib to compress better.

  2. Convert image to PNG8 using pngquant.

    It reduces images to 256 colors, so quality depends on the type of image, but pngquant makes very good palettes, so you might be surprised how often it works.

  3. Use Zopfli-png or AdvPNG to re-compress images better.

    This is lossless and recommended for all images if you have CPU cycles to spare.

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