A quick test of the "new" Google Photos service, and its claimed "imperceivable" compression. Original is a 15MP 9.2MB file. The Google version keeps the resolution but shrinks it down to 3.2MB.
There is noticeable grain reduction in the compressed image, but it is acceptable given the level of file size. The image on the right shows the pixel differences between the two files (levels adjusted for clarity).
original: https://www.flickr.com/photos/joehigham/17234028631/in/album-72157652105733412/
via: http://midletonrared.com/2015/04/23/109961/
