Google are presumably storing/caching these photos for an hour before expiring and redownloading them. I don't see why Google don't redownload them using If-Mo

Google are presumably storing/caching these photos for an hour before expiring and redownloading them. I don't see why Google don't redownload them using If-Modified-Since, and reusing the original downloaded photos in the case that the origin server says they've not changed.