To accurately determine which body system is most represented in the picture of a frog, I would need to see the image in question. Typically, frog illustrations on title pages might highlight specific systems depending on the focus of the content (for example, educational material on anatomy). If the image emphasizes the brain or spinal cord, it would represent the nervous system. If it shows organs like the kidneys or bladder, it would depict the excretory system. If there is an emphasis on the digestive organs like the stomach or intestines, it would represent the digestive system.
Without seeing the picture, I cannot definitively answer the question. Please provide more context or details about the image.