1. Usually to hold up crossed fingers is used as a body language meaning "Good Luck!" but to hold crossed fingers behind your back is used to show your negation against what you are listening to.
2. Many children have this silly notion that if you cross your fingers, it cancels out what you say. So, technically you can tell a fib or lie as long as you cross your finger while saying it. This is what this painting is about – this little girl is crossing her finger behinher back because she is “the fibber”.