The Preview Image Height is different from the plain Image Height value. If you are looking for that information, click here: Image Height
In contrast to the actual image height, the Preview Image Height does not give any information about the actual size of a photo or picture. Instead, it determines the height of a small preview or thumbnail image that is embedded in a file. This can, for example, be a document, video, image file.
This preview image has a certain dimension as well and its height is determined by the value under Preview Image Height.