This little essay discusses turning a picture into a silhouette using Gimps custom paths tool. The orca marking are added as layers
Open the base photo. In this case, it was a already cropped photo (silhouette_base2.png

Create a new layer (background )
Create another layer Silhouette Layer
The layers should be arranged.

With the silhouette layer selected Click on the paths tool or press the letter b which will select it. The path tool allow you to very finely select a path. In this case, each time you click the mouse, wherever the the little cross-hair might be will leave a little circle. A series of clicks will leave a path of connected circles. Complete the path by pressing Control – click on the first circle you created.