You can do it like this:
% Image with alpha channel
[glasses, ~, alpha] = imread('http://sweetclipart.com/multisite/sweetclipart/files/sunglasses_black.png');
% OPTIONAL: Let's rescale it (it's very big!)
glasses = imresize(glasses, 0.1);
alpha = imresize(alpha, 0.1);
% An image of a person (let's put the glasses on the person).
person = imread('http://cinemacao.com/wp-content/uploads/2013/12/Scarlett-CAPA-2.jpg');
% Lets make the alpha MxNx3 (so we can combine it with the RGB channels).
alpha = repmat(alpha, [1 1 3]);
% And convert everything from uint8 to double (to avoid precision issues).
glasses = im2double(glasses);
alpha = im2double(alpha);
person = im2double(person);
% Final image
% Let x,y be the top-left coordinates where we'll put the glasses.
x = 440;
y = 450;
% Let's combine the images.
img3 = person;
img3(y:y+size(glasses,1)-1, x:x+size(glasses,2)-1, :) = ...
glasses .* alpha + ...
person(y:y+size(glasses,1)-1, x:x+size(glasses,2)-1, :) .* (1 - alpha);
% An display the result.
imshow(img3);
Result: