You can just index elements in bigger image and assign them values of smaller image.
combinedImage = biggerImage;
s = size(smallerImage);
combinedImage(x:x+s(1)-1, y:y+s(2)-1, :) = smallerImage;
This example works only if smaller image fits entirely into bigger image after placed to specific location!
If you want be able to put smaller image to location that part of it will be outside of bigger image, you have to handle this case properly. There are more options you can do. You can extend bigger image (and set some background color) or crop smaller image.
Whole example code:
% 'specific location' point
x = 123;
y = 274;
% create example images
biggerImage = rand(512,512,3);
smallerImage = ones(64,64,3);
% merge images
combinedImage = biggerImage;
s = size(smallerImage);
combinedImage(x:x+s(1)-1, y:y+s(2)-1, :) = smallerImage;
imshow(combinedImage);
Result:
