Unless text is encapsulated in a Text box, it is a fixed background layer that inserted objects float above, and cannot be moved behind. The one exception is the selected image that is moved to section master, where it becomes a watermark behind the text on every page of the current section.
If you want an image over text effect, you can insert your image, which is now in the foreground. Then insert a Text box that causes deselection of the image, and the Text box is now the foreground. Press the shift key and reselect the image. Visit the Arrange panel, and set Text Wrap to None. Place text in your Text box, and now you can drag the Text box over the image. Scale both to taste, and then in the Arrange panel, select Group.
Another opportunity to place text over an image is to insert either a Shape, or Text box, and from the Style panel, one can select Fill > Image Fill, and choose an image to fill the background of the object. The opacity slider allows one to tone down the image so that text entered into the Shape or Text box approaches legible, though depending on the image, it may be difficult to find a text color that will complement and standout over the image.
See the Pages User Guide for Mac.