Seigex it is really complicated -- while there may not be import duty there may be VAT/GST -- and it is so important that a work be classified properly.
One of the most unusual cases with art and VAT was a problem a gallery had importing a sculpture by the late artist Dan Flavin -- the artist used long florescent light tubes and the art works were classified as light fixtures -- with a higher VAT. UK Customs did not believe it was art and the gallery had to file an appeal.
I stick to drawings, much easier to transport, but have used customs brokers who have been worth the fee.
I know that paintings are shipped via air cargo for museum exhibitions but there is a customs broker involved.
I hope your wife has a successful exhibition and congratulations to her.