i started with an image which was put onto the background layer. im to make cracks in the image to show some of a painted layer under it. so i think i need to swap which layer is the background layer. how would i do that?
When you create a new image via the File → New then there is a layer called Background at the bottom of the layers stack which is intended to be used as a background layer.
(That layer is actually a special layer that is continuously white.)
When you do File → Open, you get one layer with the opened image on it and that layer is called ‘Background’. The layer name is just a name/label and can be changed by you at any time by double clicking on it and typing a new name over it in the layers docker.
You can do that with any layer in the layers docker. You can even give them all the same name if you like causing confusion.
If you add a new paint layer by using the small ‘+’ icon at the bottom of the layers docker, you can then drag the layer containing your image upwards to be above the new paint layer and then use that new paint layer as a ‘background’ layer for your artwork.
You might want to change the layer names, as required, to avoid confusion when working on the document.