Vegetable Gardeners of a nervous disposition avert your eyes now
The Large White Butterfly (Pieris brassicae).
The emphasis is on the latin name of Brassicae. The Cabbage is from the "Brassica" family of vegetables. These images are of a female Large White and her aim in life now is to lay as many eggs as possible.
A favourite place would be under cabbages laying a neat stack of eggs under the leaves.