WordPress was designed for public websites, not private ones, so password protection can be a little clunky. Fortunately there are plugins to help, but (as always) there are trade-offs to be made.
When all you want to do is add a password to stop search engines indexing and outsiders reading the content, but you also want make it as easy as possible for people to use, there’s the Password Protected plugin. As it says, it doesn’t protect the images or other uploaded content.
If you also want to protect the media, you will need to give people an account on the WordPress site (with username and password). Then you can use the htaccess edits detailed at http://www.idowebdesign.ca/wordpress/password-protect-wordpress-attachments/. This works, but in many cases you just don’t want to give lots of people accounts on the system, or make groups of people share an account. So it’s a trade-off – how important is password-protecting the images versus the administration overhead of user accounts with the associated username/password ease of use issues? If you do want to use usernames and passwords, perhaps giving a group of people a shared account, I’d recommend also using one of the plugins that helps with finer-grained access control, such as Members, to stop people being able to change things you don’t want them changing (such as passwords for the shared account).