This is one of the most important decisions your going to have to make, so don't rush through it. You'll probably going to find more than one draw-worthy photo so I would go ahead and create a new document on your computer to store all of them.
Now there's nothing neccasarily wrong with just typing the breed of dog you want to draw into google images and clicking on the first picture you see, but that usually doesn't work well. Your going to want to pick something that YOU can draw. Just cause' it would look good as a drawing doesn't mean it's within your skill level. Try to find a drawing with an interesting composition, good contrast, detailed enough to print, and uncluttered.
Below you can see the drawing I chose to use for this instructable. I chose to draw a Siberian Husy for several reasons. First off, you can see that the husky puppy doesn't have short fur but it's fur insn't overwhelmingly long, keep in mind when your choosing your photo that the longer fur is the harder it may be to draw. You can also see it's fur has a LOT of contrast. Since the husky is basically black and white it's very easy to see right off the bat how your going to shade it. This particular photo also has a very good composition, the dog has it's ears upright, a friendly expression, and it's tongue hanging out and eyes gives it a strong personality. You can see all four paws though it's sitting down, you can also see part of it's tail. Also, you can see there's not a whole lot going on in the photograph, that will make it easier for you to isolate the figure and focus on drawing the dog withought having to be distracted by other elements of the photo.
So, go ahead and choose the photo for your drawing. Remember, choose wisely!!! The last thing you want is to start a drawing and get bored with it halfway through, or get fustrated cause' it's to difficult. Once you pick a photograph, print out a GRAYSCALE image of it. Don't print it out really big and compenscate by making it blurry or anything like that.