Skype recent pictures folder on MacOS High Sierra
~/Library/Containers/com.apple.ImageKit.RecentPictureService/Data/Library/Images/Recent Pictures/ is where Skype stores your previously uploaded photos in this version of MacOS. It changes all the time so might not hold true for future OS updates.
Quit Skype, delete photos that you don’t want anymore, clean from trash, restart your Mac.
If you open Skype before cleaning trash or restarting then they will be copied again.