Copyright - US Law gives the copyright holder (not always the creator of the text or image) the rights of distribution, reproduction, display, and derivative works. You do not have an inherent right to use content, even if it is "for educational purposes."
Plagiarism - Implying that someone else's words or images are your own is plagiarism. Crediting sources prevents plagiarism, however you still need to be concerned with copyright when reproducing images in presentations or posters.
Today, images can be easily cropped, resized, colored, or otherwise modified using a variety of software. Be aware of your image's properties. File size and dpi (dots per inch) will affect the final product of your presentations. Before printing a poster, be sure to view your images at 100% and check them for pixilation.