Origins of the picture:
The above photograph (usually credited to National Geographic magazine or Geo, a similar German-language publication, and mailed under titles such as "Really Bad Day!" or "Jaws 2001" or "Photo of the Year") first appeared back in August 2001. The image created by combining two real but unrelated photographs — a picture of a great white shark taken by South African photographer Charles Maxwell, and an Air Force photo of an HH-60G Pave Hawk helicopter from the 129th Rescue Wing, Moffett FAF (a California National Guard unit) snapped by Lance Cheung (with San Francisco's Golden Gate Bridge in the background). This faux image is also often circulated in combination with an unrelated photo of a surfer and a looming shark which, although real, shows a surfer and a dolphin, not a shark. From snopes.com
Copy and paste the helicopter picture into GIMP. These pictures are smaller than what you are used to. If you want them to appear larger you will have to zoom in.
Use the flip tool on the toolbar to flip the picture.
Copy the shark picture and paste it onto the helicopter picture.
Use the scale tool to make the shark smaller.
Use the move tool and the eraser to blend the pictures together.