We can use the same idea that we used with chromakey to average the colours in images (or indeed combine them in other ways). 

Here are the two pictures we are going to average:

Here is the code we will use  (Note that for this code to work effectively the original images must be exactly the same size):


script

And here is the result of averaging the RGB values of the two pictures (Fire and Ice):

fireandice 

Here are some more examples:

wedding2 wedding1 pic
     
wedding1