Compress image files to make site speedier

About 4.8MB has been saved due to compressing images.

All images have been losslessly compressed using [ImageOptim](https://github.com/pornel/ImageOptim) version 1.4.4.

Additionally, some PNGs with few discernible colors have been converted from true-color to limited-palatte. This further reduces the size of the image without any perceivable difference in quality.