Letterboxd and TMDB are two different things, we can't add a backdrop to Letterboxd. You have to contact the Letterboxd crew, since backdrops aren't imported automatically. (Go to the movie entry on Letterboxd, click on the flag and make an Image Request)

How can I add or contribute backdrops to my films? I'm new and can't find that option anywhere. Could you please help me?

@EduRonaldo said:
How can I add or contribute backdrops to my films?
I'm new and can't find that option anywhere.
Could you please help me?

Add/Vote - Movie/TV Show - Backdrop/Logos/Poster

Before adding an image read the rules in our Bible
https://www.themoviedb.org/bible/image

This operation cannot be done by Cell phone or Pad.
In Seasons it is only possible to add Poster.
In Episodes it is only possible to add Backdrop.

On the Movie or TV Show page, click Media.
Then click on Backdrops or Logos or Posters.

To Vote, pause the mouse over the image and click the thumb up or down icon.
Only one vote per image, per user, is considered.
An image will be the main image when it has the most Likes.

To Add, you will see on the left side, a small " " sign in the line of the word Posters or Backdrops.
Click on this sign and follow the instructions.
Backdrop and Poster only in .JPG
Logos only in .PNG or .SVG
Just drag the image from your file manager and drop it there.
Or use the Select file... button.

Only Moderators can delete images.

Thank u so much 🙏❤