This website, created by three American science professors, provides a comprehensive, yet concise, overview of how to create effective posters from defining your message, to planning your poster, to your poster's aesthetics, to how to effectively present your poster during the session. If you are only going to visit one resource on creating your poster, make it this one.
This webpage, by former evolutionary biologist and current photographer Colin Purrington, reads like a long, humourous, but exceedingly useful essay on poster design, both good and bad. He offers novel ideas for attracting attention to your poster, as well as a number of PowerPoint and OpenOffice poster templates.
This webpage by Michael Alley, an Associate Professor of Engineering Communication at Penn State, is short on text but long on examples and PowerPoint templates.
This webpage by Fred Stoss, Science Librarian at the University of Buffalo has some great poster examples.
Includes some excellent Poster Samples.
From the Journal PLoS.
Poster Blog - Better Posters
This short video from the American Chemical Society is a good introduction to creating effective posters.
This video teaches you how to present your poster during a session.
Make sure to give credit for images by citing them using the same citation style as the rest of your poster.
Google Advanced Image Search allows you to limit results by usage rights. Choose "free to use or share" to make sure you may use the retrieved images on your poster.
All of the images on Wikimedia Commons are freely useable. For more information, please visit this page on reusing content outside Wikimedia.
Many photographs on Flickr are available for use under a Creative Commons license.