At the moment, we do not have the option to delete a single media item from a chapter directly.
However, you can remove all images and videos from a chapter by deleting the Media Keyword for every sentence from the Edit Script section.
Note: If you want to change how long a media clip shows in your video, you can do it from the Edit Media section as shown here.
You can follow #Step2 as a workaround if you don't want the media to preview in your video.