This is often incorporated into the work and then built upon. Such manipulation and use of secondary sources still attracts the protection of copyright. Where the author is employed to write say a web page then copyright belongs not to the author but to the employer. This is a very real problem for web developers. Graphics can include computer generated shapes, photographs and even animated graphics.
Graphics are freely available on the web.
Some are free, some are for sale. The first rule is always check the conditions before you use other peoples graphics. If you do purchase graphics check the license agreement. This agreement may have limitations on the use or any adaptation of the graphic. The purchase of graphics may not give you the right to publish the graphics.