As with research papers, it is important to give credit to sources referred to in an academic poster, i.e. books, articles, newspapers, websites, images, etc.
Refer to the citation guides from the link below for examples of how to cite different types of sources in different styles.
Generally speaking, choose the citation style for the discipline your poster represents. For example, with a science poster you might use CSE style; for the disciplines of Education or Social Work, APA; for History, Chicago style, etc.
Important: If you have any doubts about which citation style to use, consult with your faculty supervisor.