Here is a MATLAB implementation of the Poisson Image editing algorithm from . It allows to seamless clone a source region (from image 1) into a destination region (in image 2).
E.g, you can place a bird in to the sky…
or put a cow in a soccer field!
Download the code here, and run the two demos to obtain the above results!
 Pérez, Patrick, Michel Gangnet, and Andrew Blake. “Poisson image editing”, ACM Transactions on Graphics, 2003.