I am searching for a book I read as a child/; young adult. It was a fantasy book and quite scary at the time. I have a strong memory of a particular scene in the book. A woman, like a witch, had come to stay and look after some children, but the children thought there was something strange about her. She paints a picture of their uncle and when she has finished the picture, he disappears. The children realise he is trapped in the picture. They know there is something strange about this woman, but cannot convince anyone else. There were some evil people/creatures around they were supposed to be wary of and they think she is one of them. (called baugrims or something I think).
Ring any bells anyone? Would be most grateful if anyone out there knows the book and can put me on the right track. It has bothered me for some time now and I cannot recall the name or the author.