These websites have generally released their photo collections under the creative commons public domain license and attribution is not generally required.
That being said, as a precaution, I would still advice you carefully check each and every photo downloaded online (and their websites), to ensure you do not run afoul of copyright laws.
This is one of my favourite sites for free images. You can find lots of amazing high-resolution pictures here. Images are free of copyrights and are released under Creative Commons CC0.
Here you will find awesome and geeky high resolution photos that have been photographed by Ryan McGuire. They are free for use without attribution.
This is a free stock photo site created by Viktor Hanacek. There are tonnes of high-resolution pictures that you can use for free.
This site is loaded with stunning pictures that can be used for whatever you want free of charge. No attribution is required.
This is another great site for free stock photos that you can use however you like!
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