Every document or image available via the Internet, unless you know to the contrary, is protected by copyright. The form of distribution (print, airwave, or computer network) makes no difference in the copyright protection provided to a work. The fact that a copyright holder elected to share his work over the Internet does not mean that anyone can freely appropriate it for other uses.
The technology of the Internet makes copying an item very simple. It is easy to adapt, modify, resend, forward, copy or display an item. Copyright law still applies, as do the fair use exemptions.
Links to a site, as long as they are to the main page, are not a violation. Remember, permission can always override any limitations imposed by law and/or guidelines.