Atticus will automatically add bleed to any book that include a full page image or a background image in the theme settings.
You will want to make sure the image you upload is properly sized before importing it into Atticus, however. This post offers a handy calculator to help: https://www.atticus.io/calculate-image-size-for-print-and-ebooks/
Once a full page image is loaded into Atticus, Atticus will always export at the trim + bleed PDF requirements.