As the University of British Columbia states:
The Copyright Act does not specify any citation requirements beyond the source of the material used and, if available, the name of the creator (ie:Creator, Source). While there is no legal requirement to attribute works in the public domain to their creator(s), doing so is an important part of maintaining academic integrity. Generally, image citations should meet the same requirements as a text citation; that is, a reader should be able to find the source of the image, and the image itself, based on the information in the citation.
A basic Google search is the go-to for finding quick and easy information. But pulling random images from the Internet could be a violation of copyright law. Below is a list of places you can go to find shareable media to enrich your online course content. Always be sure to find out if there are any restrictions on reuse, such as an attribution requirement.
A screenshot showing how to filter Google Images results by usage rights
The Visual Communication Guy has created this quick and easy infographic for when you're just not sure if you may use an image in your work.