you can read in your two images using imread - this example uses jpg but bmp, tiff, png, etc work also and make sure the dimensions of the 2 pictures in pixels are the same
file1 = imread('picture1.jpg');
you now will have 2 matrices in file1 and file2
now just form the sum of the 2 matrices
you can weight one of the files more than the other by multiplication, as shown here using 0.2. You can try different values of the multiplication until you get the image you want. some give very pretty results. check out lots of values.
it is exceedingly simple to do and the results are very nice.
have success, let me know how it went.