Copyright is tricky, but to make it simple: If you didn't either:
- Make the image, take the picture, write the text, or otherwise "create" the content
- Get explicit permission to use the content created by someone else
... then it shouldn't be copied and/or posted.
There are a lot of nuances around changing images, etc., but one of the biggest issues online with copyright is that people think "If it's posted on the internet, it's fair game." Even without a watermark, copyright symbol, or text saying "do not copy", there is (at least in the US) an implied and enforceable copyright.
There are lots of sites that have images that they expressly say something like "free to use", or say something about "creative commons". Here's an example: https://www.flickr.com/creativecommons/
You can search useable photos at http://search.creativecommons.org/
or using the advanced search on google https://support.google.com/websearch/answer/29508?hl=en
Many of the images on flickr also have a commons license with some rights reserved, requiring attribution and/or a link back to the original post.
Ok, thank you for clearing that up!