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Yeah, I think it's safe to say that you probably can't use them. Where did you find them, Google image search?

You have an option there to only look for images 'labeled for reuse'. Those are most likely images that you can use commercially.

Here's a screenshot:

@daan your an absolute star, this is exactly what I was looking for! I checked this option and there's a load of images I can use. I'll still add the sources and mention in my article about the 'labeled for reuse'

It's better to check the individual license on the page from where the image originated. If it's from Wikimedia, it's easy, you can just check the exact license on the page.

Most of the time, they're licensed under public domain, you'll see many images from Wikimedia, Pixabay and maxpixel.net :)

That's really useful to know, thank you.