\uD83D\uDCD8 Answer
The images that children are asked to name were derived from an evaluation of vocabulary words within multiple pre kindergarten curricula. While some of the words might be considered to be more difficult for children to identify, items were included that mapped onto content presented in commercially available curricula. Pictures are randomly shown and are in ranges of easy, medium, and harder difficulty.
More information can be found starting on page 20 of the CIRCLE Progress Monitoring Technical Manual.