Use the Fade Background dialog to set the visibility for the background picture.
Click the Background Visibility button on the toolbar, then choose Fade Background in the pop-up menu.
Alternatively, Control–click and choose Background Visibility > Fade Background.
Drag the slider to set the background visibility to any point between 0% (background off) and 100% (background fully on).
As you drag the slider the appearance of the background changes.
Click Close when the background is at the desired fade level.