DT Register does not have an image that loads for each event. You can use HTML in the Description field of DT Register to load an image there. Be sure to watch our video tutorials to see all of this type of thing.
Since you said "calendar" I'm wondering if you're talking about loading an image in JEvents. There, you can have default COLORS for event categories, but not default images. Again, you can load images into the description of the event for view in the calendar.