[ImgBot] Optimize images

*Total -- 665.90kb -> 483.12kb (27.45%)

/img/post/thumbnail_news.png -- 158.61kb -> 105.55kb (33.45%)
/img/mainvisual-mobile.png -- 231.55kb -> 168.90kb (27.06%)
/img/mainvisual.png -- 275.74kb -> 208.67kb (24.32%)

Signed-off-by: ImgBotApp <[email protected]>

Author: ImgBotApp