[ImgBot] Optimize images

*Total -- 953.12kb -> 685.55kb (28.07%)

/img/howto-highlightImageBlack.png -- 811.53kb -> 562.29kb (30.71%)
/img/howto-highlightProperties.png -- 11.67kb -> 9.97kb (14.59%)
/img/howto-highlightTextBlack.png -- 51.81kb -> 44.54kb (14.04%)
/img/howto-highlightTextDefault.png -- 78.10kb -> 68.76kb (11.96%)

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

Author: ImgBotApp