When using images created by others, it is imperative to use the images ethically and with attribution. It is your responsibility to check the licensing agreement on each image you use, and to cite or attribute each image to its creator. Refer to the Citing tab of this guide for instructions on how to cite properly.
Try an open-source reverse image search using TinEye or Google's Reverse Image Search. Both places let you upload an image, refer to a URL, or download a Firefox or Chrome extension that allows users to right-click on an image.