[ImgBot] Optimize images

*Total -- 3,303.62kb -> 2,914.25kb (11.79%)

/images/hangman_thumb.png -- 34.43kb -> 14.81kb (57%)
/images/login-background.svg -- 1,686.68kb -> 1,401.43kb (16.91%)
/images/cool_bg.jpg -- 1,582.51kb -> 1,498.01kb (5.34%)

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

Author: ImgBotApp