[ImgBot] Optimize images

*Total -- 846.05kb -> 788.50kb (6.8%)

/images/screenshot.png -- 394.00kb -> 350.79kb (10.96%)
/images/tn.png -- 173.74kb -> 160.16kb (7.81%)
/assets/images/gopher.png -- 278.31kb -> 277.54kb (0.28%)

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

Author: ImgBotApp