Whether you are using an image in a paper, poster, or presentation, you must give credit to the creator (author) of that image. If an item has a creative commons license, be sure to understand the requirements for attribution.
Here is an example of how a photo retrieved from Flickr would be cited in a reference list (APA format):
Viswanathan, N. (2009). Dogs! [Photograph]. Retrieved from https://www.flickr.com/photos/nehavish/4073675070