Use Fill Transparent 
to fill any transparent areas of the canvas with the selected color.
To use Fill Transparent
-
In the Paint Tab
of the Design Panel 
, choose Canvas 
> Fill Transparent
. -
Choose Paint > Canvas > Fill Transparent.
Fill the Background to a Picture
-
Use Pick Color
to select the background color from the picture on the screen. This selects the color that you will fill with. -
In the Canvas pop-up menu, click Fill Transparent
. -
Any transparent areas around the picture, or within the picture, are filled with your selected color.