See Expression Elements for a full list.
Expression Value: Bitmap Transparency _alphaTransparency&
Expression Element: Built-in Properties
Bitmap Transparency
Returns a raster item's transparency (or alpha blend factor).
Expression Syntax: _alphaTransparency&
_alphaTransparency& returns a raster item's transparency (or alpha blend factor).
To change the Bitmap Transparency right-click on the row and select Set Transparency...
The Bitmap Transparency dialog will be displayed to allow you to change the value. 
Related Topics