Good quality graphic for series - free design costs - sounds like a winner.
A side note: A great way to inspire more variety in stamp design is to have the particular year series carry a second title. eg. "Washington State Series 2013: [insert clever series moniker here]"
"Race to [insert location of finale]"
etc, etc, etc.
This is the part that could give each year its personality and flavor.
Otherwise, year to year submissions will follow along the lines of Apples, State shape, A George Disc Head,...
in a not so western or eastern centric, general blah blah kind of way. Give the artists something to run with - fresh each year. Most artists might feel confined to give you something not too Seattle, not too Spokane... and you'll end up with - Ellensburg each time.