Unsplash (https://unsplash.com/) offers free “do whatever you want” high-resolution photos with no attribution. They have their own “Unsplash” license that essentially says you can do whatever you want, so long as you do not try to sell them via a similar service.
If you are using Unsplash for mockups, check out Lorem Picsum (https://picsum.photos/). It is a placeholder image tool that can randomly serve images from Unsplash at different dimensions.