[ImgBot] Optimize images

*Total -- 81.02kb -> 65.87kb (18.7%)

/images/insertion-sort.png -- 22.09kb -> 13.28kb (39.86%)
/images/bfs.png -- 7.03kb -> 4.76kb (32.25%)
/images/bubble-sort.png -- 13.68kb -> 9.74kb (28.81%)
/images/dijikstra.png -- 38.23kb -> 38.09kb (0.36%)

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

Author: ImgBotApp