Where you get your content is as important in producing a website as making it look good. If the client does not provide you with specific content and you do not create and sell rights to it yourself, you will likely get images and other items from various sources. You need to protect yourself, as well as your client, by using content right.
Owners/creators have legal property rights over creations of the mind, both artistic and commercial. Under intellectual property law, owners are granted certain exclusive rights to intangible assets such as musical, literary, and artistic works; ideas, discoveries and inventions; and words, phrases, symbols, and designs. Look it up!!
Know where your content and images comes from!!
Per Wikipedia, "The public domain is a range of abstract materials—commonly referred to as intellectual property—which are not owned or controlled by anyone. The term indicates that these materials are therefore "public property", and available for anyone to use for any purpose... a work may be in the public domain if no laws establish proprietary rights over the work, or if the work or its subject matter are specifically excluded from existing laws."
Licensing means you purchase the right to use an image, text, pdf, etc. under terms set out in a license. The rights are set out in the license, a legally binding agreement between the buyer and the seller.
"Royalty-free" means that you pay once for an image that can be used (but not passed on to anyone else) for multiple purposes over an unlimited period of time. You still need to be careful how you use it. For example, if the image features recognizable people and you know of no model release, then do not use the image for promotional purposes.
Traditional Usage License
Rights-managed images are sold with the fee based on your specified use. This fee is calculated based on size/amount, placement, duration of use and even geographic distribution.
This includes terms restricting the photographer or writer from selling the item for a specified period in a specified location(s). This gives the buyer exclusive use.
In addition to paying close attention to usage responsibilities and intellectual rights, a web author must use common sense in where s/he gets content, how s/he uses it, making disclaimer additions when needed, and in giving credit as appropriate.