When cleaning art its always best to remember a very mild abrasive is needed. I'm taking dishwashing detergent on a wet cloth. You'll guage your cleaning efforts with the lightening of the overall yellow hue....never anything oil / acetone/ alcohol based , Cleanse with soap first, continue to monitor painting and lastly use Windsor and Newton picture cleaner. (Just went through this with my neice)--- if its really bad send it for restoration; because while the cost is high, the results will be pristine
