Open In Fireworks

OK, Open up your sky picture in Fireworks, then Copy and Paste the sky onto your hills image. The sky will cover up the hills (not a problem) and your Layers panel will look like this:

If you can't find the layers panel, click Window > Layers (keyboard shortcut is F2) and look on the right side of the Fireworks window.

Now let's get the hills on top of the sky! Just like Flash, you grab the layer name and drag up or down to swap layers. Your Layers panel should like like mine:

And your working image should look like this:

Finally, let's name give those two layers some appropriate names. Again, it's just like Flash where you double-click on the layer name to rename it - call the sky layer sky and hills layer hills: