[ImgBot] Optimize images

*Total -- 3,969.34kb -> 3,407.87kb (14.15%)

/archive/2019/images/backend.png -- 932.20kb -> 781.98kb (16.12%)
/images/intro.png -- 80.19kb -> 68.14kb (15.02%)
/archive/2019/images/intro.png -- 79.75kb -> 67.94kb (14.81%)
/archive/2019/images/frontend.png -- 580.98kb -> 495.02kb (14.8%)
/images/frontend.png -- 715.06kb -> 614.39kb (14.08%)
/images/backend.png -- 685.40kb -> 591.45kb (13.71%)
/archive/2019/images/devops.png -- 372.11kb -> 327.00kb (12.12%)
/images/devops.png -- 523.65kb -> 461.95kb (11.78%)

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

Author: ImgBotApp