By default, Date view shows the first photo scanned or shot on a particular date. If the default is not representative of the group of photos taken that day, you can choose another.
In Month or Year view, use the slide show controls to find the photo you want to use. Then right-click the photo in the slide show window and choose Set As Top Of Day.
In Day view, select the photo in the thumbnail list, and then right-click the large photo or the thumbnail and choose Set As Top Of Day.