Click the image layer name in the Canvas Layer Editor window.
You can only sketch on one image layer at a time. This image layer is referred to as the active image layer. In order to sketch on an image layer you must select it (making it the active image layer).
You may also need to select an image layer in order to:
See Merge image layers.
The active image layer is indicated by the button in the active view window title bar, and is highlighted in purple in the Canvas Layer Editor window.
The image layer is highlighted.
To select a layer by clicking its paint