Mention nurses in the Bay Area these days and two images come to mind: a caring, nurturing professional at a patient's bedside, and a perturbed, bullhorn carrying protester walking a picket line. By one estimate, the California Nurses Association/National Nurses United has called more than 70 strikes at the state's hospitals in the past two years. Its members will walk off the job again Thursday, waging a one-day strike against seven East Bay hospitals affiliated with Sutter Health as part of an 18-month-long attempt to negotiate a new contract.