First I fit the backpack on one of the penguins by resizing it and rotating
it so that it looked more natural. I used clone stamp and the eraser tool
to fit the penguins wing above the pack and to extend the lower part of the backpack strap.
Then I merged the backpack layer with the penguins.
I then added the penguins to the background image and sized them down using the Transform tool so they would look about the right size.
Finally I added some gaussian noise to make the penguins look more natural, and scaled the brightness down to adjust for the flat light.