Images that you have found online are likely to be protected by copyright. Using copyrighted material without permission is illegal.
The links that you will find below offer access to websites where internet users share links to images that have been published freely online with the permission of the rights holder. You are allowed to use these images, provided that you meet the given licence terms.
Flickr: Creative Commons - Attribution
More than 31 million reusable photos, provided that you meet the given licence terms.
Rijksdienst voor het Cultureel Erfgoed
This image archive contains over half a million images from Dutch monuments, archeology and landscapes of the last 150 years.
Open Clipart Library
A collection of thousands of clipart images that can be (re)used without any restrictions.
Pixabay
A large collection of photos from the public domain, as well as the possibility to add your own images.
For more information about licences, go to Copyright: Creative Commons licences.