If you are unable to use an image you created yourself, it is best to use images that are in the public domain to avoid violating someone else's copyright. If you cannot find a suitable image in the public domain, search for creative commons-licensed images. Always make sure that you meet the terms of the license for any creative commons material that you use.
It is best practice to cite the source of images you use, even if they are in the public domain or don't require attribution.