You can get your photos from google for your classes. If you are using this photos for a project that lots of people see, then you should contact the owner. If only a couple people see the project then just make sure to include the watermark. You should always give credit to the owner, you must always keep a watermark on a photo, or leave a link to the photo. I would allow someone to use one of my photos as long as they left my logo on it, or a link to the actual photo. My logo is a typical camera with my name reflecting in the lens. It was easy thinking of all the colors to use behind a white and black background. But it was a little hard finding all the shapes to use in order to make my logo.