Most images are not online
Due to copyright restrictions and other factors, only a tiny fraction of the world's images have been digitized. The majority of the images needed for research purposes are found in books, magazines, or photographic image archives.
Locating print sources and scanning is often the best way to find exactly what you need. Copyright generally allows one copy of an image to be scanned for research use and classroom presentation.
The College Art Association's Code of Best Practices in Fair Use for the Visual Arts (2015) provides practical guidance in applying copyright doctrine in art practice, writing, teaching and online use of images.