The image is built around the Dish Name field in the modal, so it cannot be empty – type the meal's name (it is pre-filled from the meal, so this usually happens only if you cleared it). Meal ID and Dish Name are required means the meal has not been saved yet; save the meal first and reopen it. If the picture is generated but you get Gallery is full (max 9 additional images), choose Main Image (replace) or one of the Secondary #N (replace) options under Save Image To.