[ImgBot] Optimize images

*Total -- 182.91kb -> 169.87kb (7.13%)

/service/wechat/img/wechat_usage.png -- 14.57kb -> 7.69kb (47.21%)
/service/wechat/img/wechat_sandbox_2.png -- 68.20kb -> 62.27kb (8.7%)
/assets/gopher-letter.svg -- 100.15kb -> 99.91kb (0.23%)

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

Author: ImgBotApp