Look at the yearbook picture and decide what you don't like about it. Think about whether the picture is not good because of how you look, the background, or if it was the fault of the photographer. If the quality is not good, do not blame yourself.
Beg and plead. You may be able to get your picture changed if you can have another taken before the yearbook goes to print. Ask the school principal or yearbook advisor so that you have a better chance of getting your new picture.
Call the portrait studio or photographer to see if they can re-shoot your portrait session. This is especially important if you notice that the quality of the photograph is poor. If you are just unhappy with blemishes, ask if the photographer can Photoshop your pimples out.
Forget about how bad your picture is. If it is too late to change your yearbook picture, just deal with the situation and remember that it could be worse. Most likely, the picture will be looked at a lot when the yearbooks are handed out, but after that, it will just sit in another book pile.
Remember that your yearbook picture will not be viewed by all people as being bad. Everyone has different opinions and some of your friends may actually like your yearbook picture.