Improve image compression

[XhmikosR]

Improve image compression

Used

oxipng -o max -Z --preserve -s --fix --alpha -r --zi 255 .

and

zopflipng --lossy_transparent -y --iterations=500 --filters=01234mepb --lossy_8bit input.png input.png

The SVGs are the same mostly, apart from beautifying them
and using the latest SVGO 3.x build.

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