When you add text boxes and shapes to an image, they’re added in layers corresponding to the order in which you created them. For example, a rectangle created after a text box will obscure the text if you position it over the text, which may not be desirable. Other graphics apps tend to have Bring Forward and Send Backward commands to rearrange objects, but Preview lacks any such controls. Here’s the workaround: Option-drag the bottom shape to create a duplicate, which is newer and thus will appear on top of any other object. After making and positioning the duplicate, you can delete the original object.