Open a terminal (Ctrl+Alt+T), and type in the following:
gedit ~/.config/user-dirs.dirs
This will open the file user-dirs.dirs
in a text editor. Look for the variable XDG_PICTURES_DIR
and make it point to your Pictures folder. So for example, if you've recreated the Pictures folder in the same place it used to be (which is in the Home folder) then you should edit that line to look like this:
XDG_PICTURES_DIR="$HOME/Pictures"
Note: it's case sensitive. So if your folder is "pictures", the directory would be $HOME/pictures
.
Save the file and close the window. Close Nautilus (the file manager) if you have it open, and reopen it. You should see your Pictures folder in the links on the left, and you should also see the icon of the folder change to its default one.