Do your students simply do a Google search for images and download whatever they find regardless if they have the right to use the image. Google has for awhile allowed one to do an advanced search and specify only images that are free to reuse, however this wasn’t a simple and obvious method to search.
As of today, Google has moved that filter to the main image search page. For example, I did an image search for Pandas.
You can see my search bar and now I can select Search Tools and then Usage Rights. If it is labeled for reuse, then you can use it in your student’s work – assuming you’re not charging money for the revised work.