Finding images on the Internet
Looking on the Internet for images to use for projects, whether for students or for teachers can often be problematic, but there are some fantastic sites available for use.
One of the best sites for Public Domain images that I found is unsplash has a great set of attractive photographs that can be used for any purpose (even commercial so selling lessons on TES is completely fine).
One of the most ways to find images that you can use (even for commercial work) is to use Google. Image search allows you to look for images with alternative usage rights.
Once you have searched for an item you can click on the tools icon, and select the usage rights that you desire. However you should be aware that the system provides a false sense of security and does not always work, and that you should check the image on the original website wherever possible.
As an author, or posting work on the Internet (even if it is not paid content) you do need to be careful around the notion of usage rights as there are punishments for violations. The 10% rule for educational use of copyright material (which would not apply for paid work, or that posted on the Internet as is made clear), is itself is a myth.
Which brings us to the confusion around conflating copyright cleared materials, the public domain and attribution (SWGFL do not distinguish all three with their copyright free material for the classroom work).
Some materials are in the Public Domain, and can be used for any reason. You don’t need to reference who created the work, or do anything apart from use the images.
Some authors allow you to use their work, but ask to be recognised (attribution).
Some work you can use upon payment (Shutterstock providing images for national newspapers and the BBC).
Some great links
Along with the previously mentioned unsplash, I use some great sites for images.
I love the more complex images on freepik, but these must credit the author. This means that text recognising the original author has to be added to your final work (which may not be ideal).
People often feel that files on flickr are in the public domain; no so since each image has a different set of rules to follow upon publication