You may use other images in your book trailers IF: (1) you have the RIGHT to use the image; and (2) you provide ATTRIBUTION (authorship, provenance, date, or location) for the image.
The easiest way to ensure you have the right to use an image is to use images that are in the public domain or are available under a Creative Commons license.
You also have the right to use images that you have changed (transformed) in some way so that the image is not identical to the original. That is, your changed image could not be downloaded and passed off as the original. A cropped image and an image panned from side to side are two examples of acceptable transformed images.