If you want to use an image in your work, it is very important that you have the right to use it. When you want to use a picture that you did not draw, or a photo that you did not take, you need to choose an image that is copyrighted in such a way that you have permission to add it to your finished product.
One of the easiest (and most interesting) ways is to use a website called FlickrStorm. Click on the "advanced" link, and choose the "non commercial" license. Then enter your key word, and watch the pictures appear! Download the search page of photos you have chosen, which will include the photo credit information.
Here are 3 videos (made for teachers) by David Jakes about how best to use the site.
Introduction to searching with FlickrStorm:
Part 2 - more about searching
Part 3 shows how to make sure you have all the photo credit you need from your downloads.
This slide from Konrad Glogowski's presentation at the K12 Online Technology Conference shows an interesting way to credit a photo. Notice at the very bottom of the picture, the photo credit is imposed over the image (it's the grey writing, but so faintly that it does not detract from the picture. This is a far more authentic attribution of authorship than a long list of links at the end of a presentation.