Not only can you use graphs and images from online articles (by using a html version and saving), but you can find reliable images from some web sources. Here are some tips for quality images. Please be sure you cite them and note that most of these are Copyrighted and cannot be reproduced on the Internet without permission.
Google Images - Sort by image size or color.
CalPhotos - High quality images from organizations and photographers that usually allow educational use.
Flickr Commons public photo archives.