Opacity: Vectorize: Trace

The Opacity determines the opaqueness of the Image loaded into the background.

Adjusting this helps make the either the Image or the Drawing Lines easier to see. 

Move the Opacity Scroll Bar to the Left to make the image Dimmer.

The image becomes much dimmer, making the Drawing Lines brighter.

Move the Opacity Scroll Bar to the Right to make the image Brighter.

The Image becomes brighter, but it may make the lines and arcs you are drawing harder to see.