As stated elsewhere, all material created and existing in a tangible form is copyrighted. Material on the web is copyright protected just as paper material is.
When in doubt, seek permission for the use and/or look to the fair use exceptions written into the law or link to the material.
The video, "Fair Use is Your Friend", from the Center for Social Media, discusses how a fair use exception can, and they feel - should - be interpreted broadly. The video suggests that it is up to the public to push fair use limits to their fullest rather than pull back and lose the opportunity of using materials for projects that could be considered fair use.
Another short film, "Freedom of Expression" also talks about copyright, the changing role it's playing in a digital age, and the responsibility we share to ensure the continued ability to use copyrighted materials.
Ultimately, it's up to each individual to decide how they wish to proceed when using copyrighted materials. The law allows a certain amount of leeway but each person has a responsibility to understand copyright and fair use.
It's always smart to have asked for permissions and/or to have considered your fair use arguments prior to using materials. However, if a copyright holder objects to your use of his/her work, you will have to remove the material from your website, blog, book, article, etc.