If you want to use images found on the Web for teaching purposes, you may do so under the 'fair dealing' provisions, but remember that you are restricted to using these images for teaching purposes only.
Under the Statutory Education Licence, Australian educators can copy and communicate text, images and print music without a copyright clearance if:
The Statutory Education Licence allows the use of text and images from any source or format:
Educational institutions that are covered by a remuneration notice with the Copyright Agency are permitted to copy and communicate text, images and notated music subject to certain conditions.
These images can be communicated for teaching purposes in presentations for lectures and uploaded to the Learning Management System (Blackboard), as well as be used for research or study by current staff and students. The amount you can copy from these works isn’t specified in the Copyright Act, but Fair Dealing applies to these and other types of works such as diagrams, artworks, films, TV programs and CDs.
However, you must provide the relevant attribution next to the photograph, or close by (eg on the edge or bottom of the page) if that is too obtrusive.
Check your referencing guide for more on attribution and referencing of images.
There are various sites that provide free access to images that either sit in the public domain or have a Creative Commons licence, like for example Flickr, Pixabay, Wikimedia, etc. See the CDU Open Education Resources Guide for more options.