Make the shrimp stock:
Remove the heads from the shrimp (to do this, grab right behind the gills with your index finger and thumb and twist off the body). Peel the shrimp and reserve the heads, the shells, and the shrimp in separate piles.
Heat the oil in a heavy-bottomed 8-quart stockpot or Dutch oven over medium-high heat. Add the shrimp heads and shells, carrots, onions, and celery and cook, stirring often, until the shells are orange and the vegetables have softened, about 15 minutes. (It may be necessary to reduce the heat.)
Stir in the tomato paste, if using, and mix well. Pour in the cold water and drop in the bay leaf. Bring the liquid to a boil, reduce the heat to low, cover, and simmer gently for 20 minutes.
Strain the mixture through a fine sieve (or a colander lined with several layers of cheesecloth) into a clean pot. Discard the solids. Place the pot of strained stock on the stove and bring to a bare simmer. Season the stock with salt and pepper until it tastes rich and complex. (I know, I know. You've been warned never to salt stock, but like with boiling pasta, this is the only chance you get to infuse flavor into the rice. And since you're not reducing the stock, there's little danger of over-salting it.)
Make the shrimp risotto:
While the shrimp stock simmers, devein and coarsely chop the shrimp. If you'd like to decorate the top as in the photo, set aside two whole shrimp per person.
Heat 2 tablespoons butter in a wide, heavy-bottomed pot over medium-high heat. When the foaming stops, toss in the shrimp, and whole shrimp if using, and sauté until light pink, 3 to 5 minutes. Scrape the shrimp into a bowl.
Return the pot to medium-high heat. Add another tablespoon butter and the shallot and cook until the shallot is translucent, about 3 minutes. Add the garlic and sauté 1 minute more. Lower the heat to medium if needed.
Dump the rice into the pot and stir until each and every grain is well-coated with the butter. Sauté the rice gently for a few moments more until it starts to turn translucent, then add the wine and stir constantly until it has evaporated.
Add the stock by the ladleful (about 1/2 cup at a time) and stir constantly-yes, constantly-until the liquid is absorbed. Repeat until all the stock is used and the rice is creamy and al dente, 25 to 35 minutes. Make sure as you stir to sweep along the entire bottom and sides of the pot to prevent the rice from sticking or scorching.
Stir in the shrimp chunks and the remaining 2 tablespoons butter and allow the shrimp to heat through. Season the risotto with salt and pepper, if you so desire, and spritz it with a little parsley. Decorate the top of each serving with the reserved shrimp by placing them at a jaunty angle. Serve immediately.