Remove unused image.
Losslessly compress images.

Before: 672 KB (688.519 bytes)
After:  527 KB (539.965 bytes)
8-bit images.

Before: 527 KB (539.965 bytes)
After:  418 KB (428.346 bytes)
Tweak images.

Make the images responsive, class adopted from Bootstrap.
Clean up vendor prefixed properties.