Images from a public domain film. Are they also in the public domain?

The images don't belong to IMDb, so the company itself can neither grant permission to copy them nor complain about them being copied. (Likely there is also no inherent mechanical copyright shared with the actual copyright holder.) We might be able to infer without certainty that IMDb would prefer that visitors not download the images, whenever the IMDb web pages upon which the images appear have disabled the use of the context menu (the little menu that appears when clicking the rightmost mouse button). Technically the images are downloaded and kept in the form of cache (possibly on the site visitor's device's hard drive for an indeterminate amount of time), just by viewing them in a web browser, but that doesn't necessarily imply the license of the site visitor to distribute them. So, ultimately, the key would be to check with the actual copyright holder, unless doing so is unnecessary or a fair use rationale is being applied. Just to note, the Amazon/IMDb company can probably tell if a copy of an image found anywhere on the web is an byte-for-byte copy of a file that was at one time or another hosted on Amazon's content delivery network (CDN), the Amazon Web Services (AWS), but the company ought not care about that. Unless mass harvesting is afoot, there is no threat to the company's business model.