Round ViewPort... or something like this

Create a rectangular viewport, cls, draw your radar images to it, then draw an image the same size as your viewport with a circle drawn with transparent colour.
*very* quick code...

Graphics 800,600,0
image:TImage=LoadImage("max.png")
viewport:TImage=CreateImage(100,100)
SetColor 255,0,0
DrawRect 0,0,100,100
SetColor 0,0,0
DrawOval 0,0,100,100
GrabImage(viewport,0,0)
SetColor 255,255,255
Cls
While Not KeyHit(key_escape)
   Cls
   DrawImage image,0,0
   SetViewport 0,0,100,100
   Cls
   DrawRect 20,20,10,10
   DrawImage viewport,0,0
   SetViewport 0,0,800,600
   Flip
Wend

You could also draw the oval with colour 1,1,1, draw the viewport image and then the radar images so you don't need the viewport cls.