This plugin seems to work well in my testing just now. The only reason it's 4-stars and not 5-stars is because adding images isn't as smooth as it could be.
To add a photo to a post, you:
1. Go to Dashboard -> Media -> Pexels Photos
2. Select a picture (this downloads it into your uploads/ folder).
3. Go back to your post
4. Insert image and find it via the media library.
It would be nice if there were either
a) a way to add a pexels photo as a one click gutenberg block (see the Unsplash plugin for an example of what that looks like)
b) a way to add a pexels photo in a few more clicks but still from the post editor. So perhaps "add image", then a new tab shows up for picking from Pexels (see Instant Images plugin for example there).
In any case, thanks to the author for creating this.