superimpose two png images with top layer transparent

%input background image

bg = imread('1.png');

%input top layer image

im = imread('2.png');

%set top layer transparency

imAlphaData = 0.8;

figure;

%draw background image

ibg2 = image(bg);

hold on

%draw top layer image

iim2 = image(im);

%set the transparency as imAlphaData (0-1)

set(iim2,'AlphaData',imAlphaData);

reference

http://www.mathworks.com/matlabcentral/answers/100086-how-do-i-superimpose-images-in-matlab