Posters, like essays, require you to cite your sources, making clear where you got your information, and creating citations for posters isn't any different than creating citations for essays. If you don't do this, you are plagiarizing!
You can have each citation at the bottom of the section for the information it's citing. For example, a picture's citation can be beneath the picture or at the bottom of the section that holds the picture
OR
you can place all of your citations together in one section of your poster
OR
you can place all of your citations together on the back of your poster, similar to having them as an extra page after your essay