Superposition of pictures

Pictures are part of a linear space. They do not form a linear space themselves because of two reasons: first of all, the color values are quantized. Second, there is a bound on the color values. Usually, the color values are stored as triples [r,g,b] where r,g,b are integers from 0 to 255.

Here is the Mathematica code which takes the linear combination of the pictures:

f = Import["donald.png"];    
g = Import["daisy.png"];
S = ImageAdd[0.5 f + 0.5 g]; 
Export["donaldplusdaisy.png", S, "PNG"];

Here is an other example from old math 21b notebooks. It uses the Blend function:

A = Import["ladybug.png"]
B = Import["rose.png"]
S=Blend[{A, B}, 0.5]