There may be issues uploading pictures from the Apple Photos library (as opposed to the older iPhoto library).
Backup and Sync has been known to upload face tile thumbnails from the Apple Photos library; the face tiles will appear in Google Drive.
There are two possible workarounds for this...
Go to your "Photos Library" location (typically Pictures)
Right click on it and select "Show Package Contents"
Right click the "Masters" folder and add a tag for it
Now go to "Google Photos"/"Preferences" and add the "Masters" folder to the list of folders to upload by accessing it through your Tags on the sidebar of the Finder prompt windows.
Go to your "Photos Library" location (typically Pictures)
Right click on it and select "Show Package Contents"
You can do the next step on the "Masters" folder as a whole or on one of the folders inside "Masters" (e.g. folder 2015)
Right click on the selected folder and click on "Create Alias": that will create a new folder that is a link to the selected folder
Remark: If you can't use "Create Alias" on the main "Masters" folder by "right-clicking", you can use the keyboard shortcut of [Command-Option-Drag it], to anywhere else, to make an alias.
Drag that new folder to /Pictures or wherever you want it to be
Now go to "Google Photos"/"Preferences" and add the "Masters" folder to the list of folders to upload
PS: information based on the discussions here: https://discussions.apple.com/thread/7063131?start=30&tstart=0
Shortened link to this page: https://goo.gl/VwFmzb