Video - Light box
Simply add a link to the video on Youtube or Vimeo to the image you would like to show as the preview. Image will get a "play" icon overlay and clicking the image will open the video in a Light Box effect. Width and height of the light box are set in the site wide in the main site template.
Video - Inline
To have a video display inline directly in the page, link to the video as per the light box method, but in the "Image Properties -> Advanced" tab add "inline" to the "Stylesheet Classes" field. The original image dimensions will dictate the video player size, so please ensure that the preview image matches the aspect ratio of the video to play, or there will be black bars on top/bottom or sides of video.