Copyrights & other considerations
A valuable part of graphic design is having a library of images, illustrations or icons to help speed up the design process when a client comes in with a job. It is not always practical to do a photoshoot or design the intricate elements of an peripheral element yourself due to time and cost constraints. It is tempting to just search the billions of images available on the internet and use these instead. The touch ups you may have to do would be minimal compared to creating the image from scratch. Copyrights and other licensing issues make this method of acquiring imagery very risky though.
Images have owners
Even though it may not be stated directly, images have owners or copyright holders. These copyright holders may or may not consent to your use of their imagery in your designs, especially if it is of a commercial nature. Imagine seeing your vacation picture being used on a billboard advertisement without your permission. The design firm would have gotten paid for the design while you received no compensation for the use of your image. As a designer you would love to have access to a wide array of images which are free to use, but you would also like to protect your work so that it is not stolen or missused.
There are alternatives
There are certain images which you can use in your designs without getting prior permission. Public domain pictures are images that are available to the general public for reuse and redistribution.
There are also various versions of the Creative Commons licence which allows authors to specify the permissions given with published material. Permissions such as whether the image can be reused for commercial purposes, be freely redistributed or whether modified images need to be shared with the public can be specified using Creative Commons.
The safest way to avoid copyright infringement and possible lawsuits is to either ensure that you have permission to use images or create your own. I see images that infringe copyrights on promotional posters all the time and this goes unpunished. Regardless of this, designing in this way is a liability and will not reflect well on you in the design industry. As much as possible I try to create my own library of files which I have created or found in the public domain for future use.