Layers are an important aspect of digital art, so it is important to know a bit about them. As the name suggests, layers are like multiple sheets of clear film, similar to those used in Anime and cartoons.
If you do the line drawing, coloring, and create the background all on different layers then the colors will not mix, which is something that can be quite useful. It is also common to draw hair, skin and clothes, etc on separate layers.
The background in ibisPaint is white but the layers that you draw on are transparent when you create them. Deleting line drawings or coloring with the eraser returns the Current Layer (the layer that is currently selected) back to a transparent state.
The order of the layers is fixed but you can drag the (1) Thumbnails (small images) up or down to rearrange the layers.
As photographs are not transparent anything in a layer below them will not be visible.
Now, drag the photograph you just imported to the bottom and set the Current Layer to 2.
If you import a sketch with dark strokes made by a pen or similar then it will be difficult to see the lines that you will trace over the top of them. In such a case, decreasing the opacity of the sketch layer will lighten the dark lines.
Beginners often forget to switch the current layer back, so take care!