I was having issues with an imported site from 3.x to Lightning.
I used this in the terminal:
I changed directory to my uploads directory, eg: cd wp-content/uploads/
And then I used this to sent the ownership to myself as the user and the local server as the group (wheel), and allow the files to be manipulated by the user or group.
sudo chown -R [username]:wheel .
sudo chmod -R g+w .
sudo chmod g+s .
I got the group from looking at another site that worked correctly. It fixed the site that was not working. Change [username] to the username for your computer. eg: /Users/[username]/Library/.