In addition to assigning proper credit for images in captions or in-text (as discussed elsewhere in this guide), you should attempt to contact the image owner/creator and ask for permission to use it.
If your project is strictly for class use, then Fair Use guidelines will probably exempt you from obtaining permission. If in doubt, please discuss the matter with your instructor.
You do not need to obtain permission to use images that are in the public domain (after 70+ years).