Image and video files set as Teaser position at the Item Images & Attachments Panel, are meant to be the reference for the item whose they are linked to. In most cases, these files appears at the top of the item, when it is displayed on the disp=single screen. Also, at the disp=posts screen, teasers are often placed between the title of the item and the
<!-- more --> mark (if any). As general rule, the final place where this files are shown lays on the skin design and its coding structure.
Note: displaying videos as teaser position will depend on the availability of any video player plugin.