[ImgBot] Optimize images

*Total -- 4,871.85kb -> 4,462.55kb (8.4%)

/assets/skin/ui-gray.png -- 7.77kb -> 5.15kb (33.77%)
/assets/skin/ui-white.png -- 6.88kb -> 4.68kb (32.01%)
/assets/png/colorwheel2.png -- 17.97kb -> 13.39kb (25.48%)
/assets/png/field.png -- 4,301.52kb -> 3,911.81kb (9.06%)
/assets/png/colorwheel.png -- 20.36kb -> 19.10kb (6.19%)
/assets/png/marker.png -- 1.15kb -> 1.12kb (2.81%)
/assets/png/Screenshot from 2022-01-04 19-17-03.png -- 516.20kb -> 507.31kb (1.72%)

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

Author: ImgBotApp