The image_display tag is a single tag that is intended to be used in tandem with image_index.
The image_display tag displays an image specified by the page URL which in turn is built by its tandem tag image_index.
To use this tag successfully, it has to be placed either inside an article which shares a common category with the images to display (thereby linking article and image categories), or in a location at the page template which is displayed without any special article context.
If this tag seems to display no image at all, it probably resides inside an article which is never rendered as it does not belong to the currently active category.
Display a single image as chosen by image_index