Images (pictures, drawings, paintings, posters, etc.) are treated by copyright law the same as texts, audio and video are treated. Images published before 1930 are in public domain, free of copyright. But note that if an image is subject of another work, the work has to be free of copyright in order to be used without permission. For example, if there is a photograph taken of the Mona Lisa painting, the photograph has to be taken before 1930 in order to be in public domain since we already know that the painting itself is in public domain.
Images in books are all under copyright protection if the book is published after 1930. Permission from the publisher is needed to use images in books. Please note that if a book is out of print, it does not make it copyright free. You need permission to use images in out of print books too.
If you want to use an image as the bases of your own creative work, be sure that your work is not derivative. It has to be transformative in order to be considered fair use and legal. If you just change the colors of an image or change its size or format (analog to digital, for example), this action is considered derivative. There was a famous case in 2011 when a person used an Associated Press picture of President Obama and changed the colors and made a poster of it called "Hope" without permission. He settled the law suit with AP just before losing the case in court.
Images you can use freely:
1. Artstor: UIW Libraries has subscription to this database of images. You can use the images in class for teaching.
2. Images in Creative Commons Archive. These are images that their copyright holders have made available to the public through Creaticwe Commons licensing.
3. Google Images: You can search Google Images for the image you are looking for. On the results page, at the top select "Usage rights" under tools. After selecting Creative Commons licenses, you will get images that you can use without permission.