When images are placed onto your canvas, they are added sequentially, in a layered order. The first image added to your page will be below the next image added - as if they are stacked.
Should you need to change the order in which the images are stacked, select one of the images. Next, click on the Align icon, then use the Order options to move your frame up or down the stack. The first and last icons will move the frame to the very top or very bottom of the stack. The middle two options will make the frame move up or down one step.
If you have more than two images on your canvas, keep in mind that two images that are positioned right next to each other are not necessarily stacked in sequence. You may have to click on the up or down arrow a few time to get the image to appear to change positions relative to the image next to it.
If you need to make an image the canvas background, there's a better way than to send it to the back of the stack and to stretch it to cover the space. Learn more here.