In the Canvas Layer Editor window, drag the opacity slider for the image layer.

To make it easier to sketch on individual image layers while referencing other image layers, you may want to reduce the opacity of some image layers while you are sketching. You can then increase their opacity when you are finished. (You may also want to reduce image layer opacity to “ghost back” an image.)

See Set brush and paint opacity.

The opacity of an image layer controls how see-through all paint on it is. (Opacity is the opposite of transparency.) An opacity of 1 means you cannot see anything underneath the paint. The lower the opacity, the more you can see through the image layer’s paint. An opacity of 0 means the image layer’s paint is perfectly clear and invisible.

Each image layer’s opacity is controlled by a slider along the right side of the Canvas Layer Editor window. When the layer is active, its opacity value is displayed below its name at the top of the editor, and can be changed through this field as well.