Copyright is a way the United States government protects the authors of "original works of authorship." These are things like music, artwork, literary work, architecture, and other ideas that are either published or unpublished.
Plagiarism is taking any of those pieces of work that are not yours and not giving credit to the people who came up with the ideas.
Fair Use law allows one to use a piece of work as long as it is for the good of society. It needs to enhance the ideas and move the thinking forward. This does not give you the write to right to profit from someone else's work. Fair use is usually applied to such things as reviews, news reporting, and teaching.
HubSpot gives great examples of times you might need to cite someone else's work: pictures, ideas, and artwork. Click here to see that article. Links to the site's home as well as links to the actual article seem to be the best way to cite the sources you are using.
**HubSpot also mentioned that even after siting, some people/marketers don't like you using their materials. If you are contacted, it is best to take the material off your blog/post.
Images:
ccPixs.com
https://en.wikipedia.org/wiki/Copyright
https://memegenerator.net/Batman-Meme/images/new/alltime/page/2
Information: https://mason.gmu.edu/~montecin/copyright-internet.htm
http://whatis.techtarget.com/definition/fair-use