The "background" layer is just a random layer, there is nothing that prevents you from putting it as your top or second or whatever layer.
The following should work:
This is just to show you that and how it works, so use a test image for this try.
Open your background image, it will be displayed as one single layer in the Layers Dialog. The name of that layer should be typed in bold
letters, this means that there isn't a transparency-channel on that layer yet (=you cannot change the transparency of parts of that layer without making the whole layer more transparent). You can easily change that by rightclicking on the layer and choosing "Add Alpha Channel". Now your layer can be erased on some parts and should show the checkerboard-pattern underneath those erased spots. Try this out, if it works you did everything correctly by now
. Now rightclick in the layers dialog and select "Add new layer", choose something like "Fill with background color" (or foreground color or any color really) from the dropdown menu. This is just to give you a new, visible layer instead of an empty one. Now drag and drop the new layer underneath the background one in the layers dialog. You should see the background image on top, and the parts where you erased something from the background image should now display the color of the second layer. If this is the case, you did everything right.
If it is not, I could give you some step-by-step images later (university calls