Hi
The position values refer to % of the screen with 0 and 0 indication that the image is central.
If you set Horizontal to 50 then the image moves to the right so it is halved (i.e you see just the left hand half)
If you set it to -50 it moves left so it is 1/2 way off the screen and only the right hand half is visible.
It's the same with vertical positioning.
100 or -100 will place the image just out of the frame and it won't be seen.
Nat