@Filmglede said:
Using Backdrops and Stills Legally for Movie Reviews
I am building a website for movie reviews and using an API key to fetch movie posters from TMDB.
I would also like to use backdrops and still images from various movies,
but I’m unsure how to handle this in terms of copyright.
I can’t find any specific copyright holders to credit when posting these images.

The vast majority of images are distributed by the producers of films and series for publicity purposes.
They hold the copyright.
But it is material distributed by them to publicize the films.
So I assume that use is free, as long as it does not harm the production, the movie.

But, since you are getting these images and data here on TMDb, then you need to give credit to TMDb as the source.
https://www.themoviedb.org/about/logos-attribution

Additionally, I haven’t been able to use the API key successfully to fetch these backdrop images.

I suggest you read the documentation.
https://developer.themoviedb.org/docs/getting-started
Spend some time browsing around, testing, experimenting, to understand how it works.

The API request for movie images is here:
https://developer.themoviedb.org/reference/movie-images

It is very important to me to use images legally.

I think I already answered this above.

For example, I wrote a short review of the movie Whiteout with Kate Beckinsale

This one?
https://www.themoviedb.org/movie/22787-whiteout

and would like to include some images of Beckinsale from the film.
How can I do this?

What we have are Posters and Backdrops, Fanarts of the Movies.
And images of Actors.
We don't have images of the movie itself, or scenes from the movie.
We do not have Frames from the Movie.
Only occasionally the Trailer that is available on Youtube.