Hi Issara,
I think the problem is that you forget to keep the montage inside the quotes (for e.g. 'montage'). I tried following code and it is working perfectly fine. Could you also please give this a try and let me know in case of any issue?
image1=imread('circles.png'); % size 256x256
image2=imread('coins.png'); % size 300x246
imshowpair(image1,image2,'montage');
Hope this will help.
