I mentioned in class the other day that you should might the copyright on images that you use in your infographic. You have some leeway here, since there's a "fair use" clause to the copyright laws in the US that covers some uses of copyrighted material in educational settings. However, going with Creative Commons images is always safe. These are images that anyone can use, as long as they attribute the creator and follow a few other stipulations that vary from image to image--like not using an image for commercial use or not altering the image in any way. See the CC license descriptions for details.
To find images with Creative Commons licenses, I recommend using Compfight to search for such images on the photo-sharing site Flickr. Just search for a term or terms (like "elephants"), then click on the settings on the lefthand side of the screen to limit your search to Creative Commons licensed images. You might also try searching among "tags only" or "all text" if your first search doesn't turn up something useful.
Click on a thumbnail to bring up that image's Flickr page. You'll see the license info for the photo on the righthand side of the page. (You may have to scroll down a bit.) Look for the "Actions" button near the top of the page. Click on "View all sizes" to select a larger size for the image. The "large" size usually has sufficient resolution for digital projects. Right click the image to save it to your computer, then insert it into your infographic.
Be sure to attribute the image to its creator! I usually use a line like this:Image: "Non-Euclidean," Derek Bruff, Flickr (CC)