If you happen to have a chihuahua dog, you should know that haircutting is an important element in the care of a decorative dog. Usually salons cut long-haired dogs of this breed. But the hair of smooth-haired chihuahuas also requires certain procedures, which can be done at home. So recently we had our pet model haircut.
As a dog groomer my advice is please please don't clip your chi's coat like the above picture. You will ruin your dog's coat and will actually make it hotter not cooler. The long hair is designed to both keep a dog warm and cool. Think of it like a field of grass, long grass waves about in the breeze because air can flow freely between the strands, but if you cut the grass short it doesn't move as it's too short. This is how the hair works on a long coat dog - air blows through the fur cooling the dog down and if it can't do so it will trap the heat in causing overheating. There's also a risk of sunburn which the long hair protects against.
On top of that, clipping breeds that aren't supposed to be clipped can really mess the coat up. The undercoat grows at a slightly different rate to the topcoat and can grow in prickly, itchy and in between the longer hairs resulting in a thoroughly ruined coat. There's a reason show spaniels are never clipped, it tends to turn their fine silky hair woolly and dense.
Chihuahuas are not meant to be clipped. Many breeds aren't but groomers do it because they see $$$ signs. If you want to help keep them cool take them to a groomer and ask them to bath and blow out the undercoat and trim out the hair on the feet. Dog sweat through their pads so it helps to keep them nice and clear. A large piece of cold stone or floor they can lie on is also very effective at keeping them cool.