Search engines, such as Google, are a common way of sourcing image content quickly, however,search engines index material on the Internet, irrespective of the copyright status. Therefore, you should be prepared to check the copyright status of any images you wish to use to be certain you can use them for your intended purpose.
Some sites may use third party copyright material unlawfully, sometimes inadvertently, however, further use of unauthorised content is infringement and anyone doing so will be liable, even if they are unaware of it.
Therefore it is important to use reputable websites, and check thoroughly for rights information next to the image or in its metadata. You should also check the website's copyright notice and terms of use.
Common locations would be in a disclaimer, copyright notice, or terms of use statement which are often found on the home page in navigation bars or links across the top or bottom of the page.
The section on image collections below contains links to sites with collections of images which may be used for some educational purposes, subject to individual terms and conditions.