Jacky,
Unfortunately, there is no way to move the ROI without redrawing it between frames. The previous suggestion involving the use of IMAQ Convert Rectangle to ROI is the best option for you. The top, left, width, and height parameters for the rectangle can be determined based on relative positions with respect to the center of the rectangle.
Regards,
Scott R.
Applications Engineer
National Instruments
Scott Romine
Course Development Engineer
National Instruments