You must seek permission (a copyright license) from the copyright owner every time you use a ‘substantial’ part of resource unless an exception applies (see the next page on permission exceptions).
If you have have found material that you would like to reuse over the internet, look for a 'Copyright', 'Licensing' or 'Terms and Conditions' page. This will detail how the material may be used or includes contact information so you can request permission.
Identifying the copyright owner can be tricky as the creator of copyright material is not always the copyright owner.
Likely copyright owner:
If you are unable to determine the copyright owner do not assume that you can use it. The above suggested list will be a good initial contact point.
Learn more about identifying if permission is required (and how to get it) to ensure you protect the commercial rights of others.