This Sea Turtle graphic is composed with the three images on the panel below.  It's an 
example of how you can blend images together to add color and sparkle to a sketch.  
It is a mixture of two photos of a lake along with a rough scribble of watery reflections.  

Both photos and the sketch contribute vitality to the overall effect.

It has distinct math - the shell is based on overlapping circles - one that reaches to the shoulders and another smaller one offset just below to form the lower portion of the shell. 

The shell shape for the baby green sea turtle is very specific. To draw it gives me this chill - like being transported 100 million years back in time.  

I proceed to work in digital space with images scanned in from my sketchbook.  I copy each turtle sketch and arranged them on separate layers over the top of the water reflections background.  

I lower the opacity setting of the turtle overlays to about 50%.  This is an easy way to pick up the ambient colors and blend the flow of the design into the interior shapes of the turtles.  

At this stage - what I call "merging down" -  the turtle sketches are barely visible since they blend or merge with the background.  

Working on a new layer, I trace over the shapes, building back shadows and highlights. I continue to strengthen the overall shapes and define the drawing. 

In this case, I want the turtles under water.  So just a few touches of shadow pop the turtles up just enough.  

Maybe you think they are hard to see, but I like the idea - naturally, they are going to be hard to see.

As a final step, I adjust the overall color and brightness to give a cleaner look.