Yes, there is a Reset Poster (or Reset Backdrop) option in the selection dialog for each film, which reverts your account back to the platform-default poster or backdrop. (Don’t forget to save your changes after invoking this option.) This approach is preferred over manually selecting the platform-default image, as a manual selection still counts as a customization, and your choice won’t update if our default image for the film is changed in future. (In situations where our default poster or backdrop for a given film is no longer available on TMDb — and is therefore not shown as an option in the selection dialog — the only way to restore that selection is via the Reset option.)
Note: The Reset Poster option does not remove customizations made inside individual lists (see above). To reset those selections, you must individually edit each list and remove the relevant customized poster(s).