For your poster, you will need to conduct research on your chosen topic, decide what to highlight on your poster, determine how to explain concepts and present information, and cite the sources you use.
When using images in your work, you have to be careful not to violate copyright. This webpage from Cornell University explains appropriate image use. https://guides.library.cornell.edu/findimages/copyright Look in particular near the bottom for the Creative Commons information.
Use the Google Image search to find images that you can use. Click on Image on the Google search page or go here: https://www.google.com/imghp?hl=en&tab=ri&authuser=0&ogbl
Put in a search term. Once the results display, click on Tools under the search box (on the right side), and then choose Usage Rights, and choose an appropriate Reuse option. Once you get to an image, double-check that you can use it.
More information on copyright is on this page.