The image is designed to fill the entire screen (so 1280x1024). How does the offset work? I have tried to use the "move at angle command". Is the offset designed to be used to fix some of the problems with that?
If you want a constantly scrolling background, you could set the offset to the Timer value. Like this:
-TiledBackground: Set image offset to Timer / 8, 0
Changing the value 8 will change the speed; lower numbers scroll faster while higher numbers scroll slowly. Also, dividing the Timer value by negative numbers will make it scroll in the opposite direction.