Q. Can I use photographs and other images for teaching?

Yes, but check the individual copyright permissions before you use them. 

Images that appear to be freely available on the Internet may not be free to use unless they have a Creative Commons licence granting copyright permission. 

You should check the Creative Commons licence for individual images as the creators/licensors can select from a variety of permissions which can affect how you can use the image.  When using any image with a Creative Commons licence you need to ensure that you credit the creator.    

You can search for Creative Commons images on Flickr and Google Images.