A violation of RCW 9.68A.070 is a felony. It can be either a class B or class C felony depending on the circumstances. The normal statute of limitations for filing felony charges is three years from the time the offense is committed. I see nothing in the RCW that would make a violation of 9.68A.070 subject to a different statute of limitations.
That being said, possession of child pornography is an on-going offense as long as the person is still in possession of the images. So the statute of limitations only starts running either at the time the person is caught and the images are confiscated, or at the time the person gets rid of the images themselves. Either one of those would start the three-year clock running.