Oct. 4
Looking at the source, there are two arrays of image filenames specified between script tags on that page. One is "IMAGES", which is referenced by the JS elsewhere, and specifies overlayed over the rectangle.The other, called EXTRAS, defined but not referenced elsewhere. It contains two entries, pointing at these images: https://madeby.google.com/static/images/tenfour/extras/002.j... and https://madeby.google.com/static/images/tenfour/extras/001.j....EDIT: oh, turns out this chap found it a while ago: https://twitter.com/hallstephenj/status/778033737040666624