If you want to place a photo on a date in an iPhoto calendar, simply drag the photo to the day where you want the photo to appear, and that photo will fill that day's square.
You can drag the photo to another day or remove it from the calendar by dragging it back to the photos list.
If a grey square remains after removing a photo, don't worry, it's just the placeholder where you previously had inserted a photo. To remove the placeholder, select it, and press the delete key on your keyboard.