Could it be that the tga bitmap isnt the same size as the resolution you are
using?

Try including this blur script, an toggle it with the B key;
Code:

---

//==================================================
// motion_blur.wdl
//==================================================
 
// Create a blank black (rgb = 7,7,7) bitmap the same size as the screen resolution to render to:
bmap bitmap_motion_blur="bitmap_motion_blur.tga";
 
// This is the panel that contains that bitmap:
panel panel_motion_blur
{
  bmap = bitmap_motion_blur;
  layer = -1;
  flags = transparent, filter;
  pos_x = 0;
  pos_y = 0;
}
 
var motion_blur;
 
// This essentially sets the transparency (alpha value) of our view at 100fps. Call with values of 0..100:
 
function set_motion_blur(newval)
{
  motion_blur=newval;

  if(newval==0) 
   {
     // No blur; shut off the panel:
     while(panel_motion_blur.alpha < 100)
      {
        panel_motion_blur.alpha += 3*time_step;
        wait(1);
      }
     panel_motion_blur.visible=0;
     camera.bmap = 0;
   }
  else 
   {
     // Blur is positive:
     panel_motion_blur.visible=1;
     camera.bmap = bitmap_motion_blur;
   }
}
 
starter mot_blur()
{
  while(1)
   {
     if(motion_blur) 
      {
        panel_motion_blur.alpha = 100 - pow( ((100-motion_blur)/100), (time/0.16) )*100 ;
      }
    wait(1);
  }
}
 
var blur_value;
 
function toggle_blur
{
  if( blur_value > 0){ blur_value = 0;}
  else{blur_value = 18;}
  set_motion_blur(blur_value);//higher = less blur
}
 
on_b = toggle_blur;
 

---