Shadows

I'm trying to create simple shadows for an Isometric 'wall'. At first I thought about drawing a skewed rect. However, I then checked the ZombieBlast game and thought I might do it by rotating the image itself.
However, I can't get the rotation right.
This is what I have so far...

Strict
Global scr_wide = 640, scr_high=480
Graphics scr_wide,scr_high
Incbin "grass.png"  'green 640,480 image
Incbin "house.png"  'iso 64*64 rectangle
Global grass:TImage = LoadImage("incbin::grass.png")
Global house:TImage = LoadImage("incbin::house.png")
While Not KeyHit(KEY_ESCAPE)
  Cls
  DrawImage grass,0,0
  SetBlend alphaBlend
  SetColor 0,0,0
  SetAlpha 0.3
  SetRotation 30
  DrawImage house,120,100
  SetColor 255,255,255
  SetBlend maskblend
  SetAlpha 1.0
  SetScale 1.0,1.0
  SetRotation 0
  DrawImage house,100,100
  DrawText MouseX()+" " + MouseY(),0,0
  Flip
Wend
End

The problem is I need to rotate it along an edge line rather than a point (I think).
Any ideas?