There are lots of fantastic pictures, video, articles and many other forms of media available on the internet.
Some of this content is freely available to be downloaded, used and edited, some of it is not freely available to be reused.
The thing to look out for is the copyright logo, which indicates that the media which it is on is protected.
To gain permission to use these images all you need to do is ask the owner of the website the media is on if you have permission to use it.