[ImgBot] Optimize images

*Total -- 118.40kb -> 87.27kb (26.29%)

/Bubble Shooter Game/Arrow.png -- 1.14kb -> 0.49kb (57.22%)
/corona cases forecasting/Results/plot3.png -- 42.86kb -> 30.91kb (27.89%)
/corona cases forecasting/Results/plot1.png -- 40.76kb -> 30.60kb (24.91%)
/corona cases forecasting/Results/plot2.png -- 33.65kb -> 25.28kb (24.87%)

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