What should you do first to change an image's ordering so it appears behind other objects?

To change an image's ordering so that it appears behind other objects, the most appropriate action is to use the Send Backward command. This command allows you to move the selected image down one layer in the stacking order of objects on the slide. When you send an image backward, you can effectively position it behind other shapes, text boxes, or images that may be overlapping.

The other options do not directly allow you to reorder the image in the way required. For instance, accessing the Layers menu is not a standard feature in PowerPoint, and while Format Options might provide tools for editing the image itself, it does not focus on ordering or stacking. Selecting the image and clicking Rearrange may seem relevant but does not correspond to PowerPoint's terminology or methods for changing the stacking order. Therefore, utilizing the Send Backward command is the specific and direct way to achieve the desired image positioning.