Why do you just want the top 50px?
Although you can't mess with the Dashboard Image, you can crop images. I don't have the NI vision documentation with me, but it is something to the effect of
Code:
Image* orig=YourCameraImage;
Image* top50pxImg = imaqCreateImage(100,50);
imaqCrop(orig,top50pxImg,0,0,100,50)