For the site I use image which find me on the special sites. I'm trying to use the legal picture. Of course a lot better if you have the designers, however, it is quite expensive. In main image only need for attract attention. No effects on search engine optimization does not have image.
On Web sites, we often use the image, but often do not know where they can legally download.
So I use images from stocks and, basically, I use a many of stocks.
Almost all drains free image is licensed by Creative Commons.
Creative Commons (CC) is a non-profit organization that created free to use standard contracts (free and non-free public license).
There are different types of license Creative Commons, which range from full freedom of use to be used with certain restrictions.
You can find pictures using search. But to find something on the Internet does not mean that it can be used free of charge.