If it’s a pic dragged on from the Photo Basket at the bottom of the editor, you can simply adjust the Fade slider on the Photo Properties dialog that appears when you click that photo.
If however it’s your main image, you can instead of fading it, use the Effects tab > Doodle tool (towards the end of the page) to paint white over your entire image. You can increase the Brush size to 100% to make the task fast. You then can use the tool’s Fade slider to make the painting fade.
Let us know if you have any other questions!