Permission from the copyright holder
Make sure you have written permission from the copyright holder to use an image or that it has come from a royalty-free source.
You can get royalty-free images by buying them from stock image websites. There are also free image website such as Pixabay and Unsplash, but there are sometimes restrictions on how they can be used, so make sure you read the terms and conditions first.
Please don't copy images from anywhere on the internet, including from search engines, as you could be breaching copyright law which can result in the council being fined or charged for the use of images.