Shipping a painting is always a bit scary, especially if the painting has to cross borders. I usually use normal couriers for international shipments and so good packaging is vital. You simply cannot overpack! More is best. You must always assume your painting could get stuck at customs, left uncollected at a cold/hot and damp warehouse and stacked up with other big and heavy parcels. So how do you protect your painting from all this?
One safe way of packaging a painting is to build a wooden crate. I have done this in the past and would do if I think the painting is at particular risk because it is very big, very expensive (and I could not afford a refund) or going particularly far away. But in most cases (knock on wood) a sturdy (double) cardbox box should suffice. So far so good.
This is how I usually pack a painting for shipping abroad: