setalpha

I don't quite get what you're asking for.
This is an example of using alphablend...

Graphics 640,480
image=LoadImage("max.png")
SetClsColor 100,100,100
alpha#=0.8
While Not KeyHit(key_escape)
   Cls
   SetBlend alphablend
   SetAlpha alpha
   DrawImage image,MouseX(),MouseY()
   If KeyHit(key_up) alpha=alpha+0.1
   If KeyHit(key_down) alpha=alpha-0.1
   Flip
Wend
End

but I'm sure you could have tested that it works OK in that situation.