There may be copyright protections on images you find online to use for your research projects. When you find an image, be sure to view the original image to identify the creator and rights information/license. Even if the image is "Labeled for noncommercial reuse" according to Google's Usage Rights filter, you still need to double check.
"Copyright and Fair Use Animation." YouTube, uploaded by Common Sense Education, 5 Sep. 2014, https://youtu.be/suMza6Q8J08
Use the links below to learn more about images in the public domain (free of copyright restrictions) or still protected by copyright law.
These resources contain millions of images on many subjects, almost all images are either in the public domain (pre 1923) and thus can be used without restriction, or made available under a creative commons license.
Google's Image Search find many images that are not in the public domain; however, you can use "Tools" > "Usage Rights" to filter to images that are labeled for reuse. Be sure to view the image on the original site to double-check ownership and copyright.
Many museums have started making high quality images downloadable without permissions. The following museums have made at least some of their images available for free re-use. You can explore a database of museums with Open Access image policies at the OpenGLAM website
Some museums restrict access to high quality images, and only allow smaller images to be downloaded. To get the high quality images you may need to request permissions and sometimes pay a fee.