Flash distresses a picture, particularly if the content is animate. Sheep take it bad. Notice, for example, that Weemix's The Sleeping Shepherdess no longer contains a single sheep. And if you're a woman who's just had a good wash but didn't know you had company, flash is a serious surprise. Murillo's Repentant Magdalen has been reduced to clutching a towel to herself as she sighs a heavenward jaysus.
Rest is the only potential remedy.
Rooms with stricken works are vacated and closed. I came across one such room a while back. An attendant stood nearby, looking dazed. If only it were that simple. You could have an attendant lurking behind every picture, they'd still find a way.