[ImgBot] Optimize images

*Total -- 6,703.33kb -> 6,451.78kb (3.75%)

/issue-19/what-the-filter.png -- 80.58kb -> 69.20kb (14.12%)
/issue-17/color-review.png -- 5.71kb -> 4.94kb (13.54%)
/issue-16/gif-16.gif -- 2,210.20kb -> 2,100.48kb (4.96%)
/issue-20/gif-20.gif -- 2,814.85kb -> 2,731.37kb (2.97%)
/issue-17/gif-17.gif -- 1,288.42kb -> 1,250.59kb (2.94%)
/issue-19/gif-19.gif -- 303.57kb -> 295.21kb (2.76%)

Signed-off-by: ImgBotApp <ImgBotHelp@gmail.com>

Author: ImgBotApp