You don't have to do anything, however the court is less likely to believe it wasn't you if you're not willing to identify that you were driving the vehicle. Assume your case went to trial, the state would have to prove beyond a reasonable doubt that you were driving. They will do that by showing the picture or video of your vehicle and proof that you are the owner. If your defense is only that it wasn't you, a jury might not believe you. If in fact someone else was driving, and you can prove it, that would be direct evidence it wasn't you and you shouldn't be charged with the offense.
Do what's in your best interest and identify the driver.
Contrary to the answer above, most traffic infractions in Washington State are considered civil, not criminal, offenses, and must only be proved by a preponderance of the evidence (51%), not beyond a reasonable doubt. Also, you'll be arguing to a judge, not a jury.
It is true, however, that you don't seem to have to name the driver. The state law on traffic cameras allows the court to assume that you were the driver unless you provide a sworn statement, either written or in court, that you were not. The statute does not require you to name the driver.
If you'd like to take a look, the laws you'd need to look at are RCW 46.63.170 and RCW 46.63.075. The first provides the rules for traffic safety cameras; the second provides both the presumption you need to overcome and the rules for overcoming it.
No. Under Washington law you do not have to identify the driver (unless you are a rental car agency or other very limited exceptions). You simply need to file an affidavit (under penalty of perjury) stating you were not the driver. Contrary to the unfortunate misinformation from attorneys not licensed to practice law in Washington, you do not have to argue the case to a jury. In fact in Washington you are not entitled to a jury trial for a traffic infraction (and you won't get one) and the burden of proof is a "Preponderance of the Evidence" not "Beyond a Reasonable Doubt."