I haven't read that tutorial in a long time. Does it tell you to create layers?
Or maybe you're referring to the z-order as layers? (A lot of newbies do, understandably.)
If you actually have layers, open the Layers dialog (bottom of Layers menu). Next you would select the mountain image and holding Shift key, press PgDn key once for each layer that you move it. You can watch the Layers dialog to see it moving.
If it doesn't tell you to create layers, then you're probably referring to the z-order, you would select the mountain image (with Selection Tool), and click "Lower to bottom" button on the control bar. Looks like
(I'm thinking possibly you clicked the "Lower by 1 step" button, and probably you would need to click it a few times. That's because each object on the canvas occupies a "z-layer". And when you import a raster image, it is placed on the top of the z-order. So it could take several times pressing the Lower by 1 step button. But Lower to bottom gets you there in 1 click.)