I created a 1024x1024 icon and File > Save for Web as PNG as requested.
But at the final stage, before compiling the binaries, Viewer builder showed an error message : "iTunes artwork error".
If I remove this optionnal icon, VB validates and creates the ipa and .zip.
The new artwork guidelines by apple that state that the big icon needs to be in 1024 are in effect at the end of July I think.
Until then, the viewer builder accepts 512px images.