If a picture has transparent areas, you can change them to opaque (white fill) or from opaque back to transparent again so that the background color or image shows through. You cannot use the opacity level to make the background semi-opaque.
Add a transparent area to a picture
If the picture does not have a transparent area, you can make one. You can create a transparent area in most pictures.
Select the picture that you want to create transparent areas in.
Click Picture Tools > Recolor > Set Transparent Color.
In the picture, click the color you want to make transparent.
Tip: Using Publisher 2007? Look for the Picture toolbar that appears when you click on your picture, then click Set Transparent Color.
The Set Transparent Color option is available for bitmap pictures that don't already have transparency information. It's also available for some, but not all, clip art.
To create a transparent area in an animated GIF, use an animated GIF editing program, and then insert the file again.
Change from transparent to opaque
Select the picture.