Hi there! When you import a PNG image with transparency, by default it's true that the image will be imported with Artrage's default canvas as the background. From there, you have a couple of options. Either choosing Cel as you did to make the background fully transparent and also remove the texture of this layer for any additional paint strokes you might make, or going to the paper settings and changing the opacity of the paper for that layer to 0% ( transparent ). This will leave the roughness of the canvas for this layer, just make it transparent.
Hope that makes sense!