Below you can find websites full of public domain and creative commons images that you can freely use in your digital storytelling. Be sure to use Creative Commons under the correct terms (giving credit to their creator and other possible limits, which are determined by the kind of Creative Commons license attached to the image.)
When attributing an image, there are four very important pieces of information that you need- Title, Author, Source, and License.
The license is the most important part. If you do not have that, assume that you don't have permission to use the image, period. The license will tell you what you can and cannot do with the image.
The source is necessary so that the image has a "paper trail" of sorts. Practically all images you find electronically will have a URL to link back to them.
The author is very important, and for most licenses you must reference the author.
The title is nice to have, especially if the author has given it a descriptive or artistic name. It is not as important as the others, but is helpful to readers to gain some more context from the image and for the author.