[Typo3] In Typo Script: How could I define the 2 level image as half trancparency ?

[stephan]

      BILDMAIN.file {
         10 = IMAGE
         10.file = fileadmin/images/saentismain.jpg
      }

      BILDMAIN.file {
         20 = IMAGE
         20.offset = 100, 0
         20.file = fileadmin/images/menu.png
         }

Thank you