However, if the installation which you need to fix is accessible via the public Internet, then you have to be careful to set your file and folder permissions correctly.
Security is not as important in this context, and you can loosen up file and folder permissions without fear. For example, when you dealing with a copy of your production site on your laptop, the laptop does not have port 80 open, and the site is not accessible via the Internet. There is no anxiety attached to this in the context of your local development installation.
You need to modify the permissions on the '/sites/default/files' directory so that it can be writable by the web server.
