Background subtraction for image

Hi. I use the im2bw method to do background subtraction but it said index exceeds matrix dimensions. Can somebody please help me with the background subtraction after the webcam captured an image? Here is the following code. Thank you.

start(vid);
pause(6); 
trigger(vid);
img = getsnapshot(vid);
stoppreview(vid);
image_resize = imresize(img, [2448 2448]);
imshow(image_resize);