Just like a research paper, a poster must include citations of articles you reference as well as images you use. Here are two examples of APA citations:
Journal Article:
Author, A. A., Author, B. B., & Author, C. C. (Year). Title of article. Title of Periodical, volume number(issue number), pages. url or doi
Haberstroh, S., Barney, L., Foster, N., & Duffey, T. (2014). The ethical and legal practice of online counseling and psychotherapy: A review of mental health professions. Journal Of Technology In Human Services, 32(3), 149-157. doi:10.1080/15228835.2013.872074
Image:
Author/Creator/Owner Last Name, A. A. (Publication date or n.d.). Title of image in italics [media type such as Photograph, Drawing]. Retrieved from url
Viswanathan, N. (2009). Dogs! [Photograph]. Retrieved from https://www.flickr.com/photos/nehavish/4073675070