As @DominiqueH points out, even when you are not commercially benefitting from the images, there are copyright issues to consider when using images from other websites.
If you really feel you need to use images on their website, I would advise contacting the website owner first and ask if you can use them. Be specific on what images you wish to use and how. Are you going to alter their size? Are you cropping any?
Other than that, using Google Images can help but I would be careful doing that. Even though they may show up as copyright free, they may not necessarily be so. I would use a royalty free stock photos site such as Pixabay, Pexels or Unsplash. That way if any problems do arise with anyone claiming copyright you have a specific source to refer to in order to claim reasonable belief that they were royalty and copyright free.