They are some fairly simple little tweaks, but they are pretty sweet nonetheless. It also appears from some of the photos published online that it does not automatically add or twinkle only the colored lights but rather just a simple flicker to the white spots in the photo.
If anything happens to those photos, these effects just breathe a new life into those ordinary winter and holiday shots that were uploaded. About Twinkle, The Google+ team says: upload a new photo of something that sparkles, which might be like a Christmas tree or it might be a chandelier, and Google+ will gift the user a version of the photo with lights that actually twinkle.
And about Snow, Google+ team says: simply upload a new photo with some snow in it to the Google+, and Google+ will gift a new version with the snow that is actually falling. The users might consider these a Christmas gift to them from the Google!