[ImgBot] Optimize images

*Total -- 8,625.92kb -> 6,361.69kb (26.25%)

/img/mizuki-at-home.png -- 4,597.64kb -> 2,840.34kb (38.22%)
/img/mizuki.png -- 793.91kb -> 636.19kb (19.87%)
/img/hi-im-mizuki.png -- 3,234.37kb -> 2,885.16kb (10.8%)

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

Author: ImgBotApp