Compress images

Compress images using
[ImageOptim](https://github.com/pornel/ImageOptim) version 1.4.3
Convert figures to limited-palette PNG

Convert `public/figures/*.png` to limited-palette PNGs containing a
maximum of 256 different colors. This reduces file size tremendously.
Convert assorted images to limited-palette PNG

Convert images that don't have very many visibly different colors to
limited-palette PNGs containing a maximum of 256 different colors. This
reduces file size tremendously.
Merge pull request #295 from pathawks/master

Compress image files to save space