graphics: optimized png graphics using matthiaskrgr/image_optim script.

Stats:
16 files changed in total
  16 binary files changed 14038 b (13 kb) -> 9762 b (9 kb) => [-4276 b (-4 kb)]
Merge pull request #329 from matthiaskrgr/optimized_graphics

graphics: optimized png graphics using matthiaskrgr/image_optim script.