Poserframes always starts with your image centered in the frame and all movements are relative to this origin position. The extreme position is the furthest the image can move from that position (left, right, up and down) and this is hard coded in the script. Available movement is the difference between the extreme position and the origin position and how the script uses this is something you can control with script settings.
The settings movement_min_long and movement_max_long defines how much of that available movement that may be used on the axis perpendicular to the long side of the image, while the settings movement_min_short and movement_max_short defines how much of that available movement that may be used on the axis perpendicular to the short side of the image. The setting movement_direction controls if the image should be moved towards the bottom right, the top left or at random. In this way, you define a range of movement and a direction of movement.
All four movement settings are effectively percentages of the available movement. You define them pair-wise: movement_min_long and movement_max_long together, movement_min_short and movement_max_short together. By setting a pair to the same value, the image will always move to that percentage of the available movement. More useful is to set a pair to a floor and a ceiling value, so the script can move the image to a random percentage between the two.
EXAMPLE ONE: movement_min_long = 50; movement_max_long = 50. The script will always move the image 50% of the available movement along the axis perpendicular to the long side.
EXAMPLE TWO: movement_min_long = 10; movement_max_long = 80. The script will randomly move the image from 10% to 80% of the available movement along the axis perpendicular to the long side.
EXAMPLE THREE: movement_min_long = 0; movement_max_long = 0. The image will never move (and thus stays centered) along the axis perpendicular to the long side.
EXAMPLE FOUR: movement_min_long = 100; movement_max_long = 100. The image will always move to the extreme position along the axis perpendicular to the long side.