When you used camera option to take a photo of yourself as user account profile photo or icon, you will not be able to copy the photo from any cache or user pictures library folder. Use the command stated in the article to retrieve the profile photo.
Assuming the picture in that Users & Groups pane is really the one you want to retrieve, then do it this way using Terminal:
dscl . -read /Users/john JPEGPhoto | tail -1 | xxd -r -p > ~/Desktop/pic.jpg
Replace your user name for john, and paste the result into Terminal. That will perform a hex dump to deposit a .jpg of your login picture on the Desktop, named as pic.jpg
Source: Mac Community > Location of user account / profile photo