From their website, it is mentioned that pictures deleted from computer will no longer be deleted from Google Photos.
It seems to be an evil way to force people to use exclusively the Google Photo web app.
Is there a workaround?
I like to take high-speed pics so that Google Photo will generate an animated Gif, but afterwards, I deleted the unused photos from these series directly on my computer. I want to do this offline, not on their slow web app.
Edit: not even close to the other question. Here, I am asking how to tell Google Backup&Sync to delete from Google Photos the pics that I have already deleted on my computer, manually. Yet, since they broke the pipe between Drive and Photos, I don't know whether it is still possible and/or if there is a hidden work-around.
Best Answer
It's worth noting that picture deletion on your computer only used to interact with Google Photos via Google Drive. That is, the steps used to be: (1) you delete the photo on your computer, (2) Backup and Sync syncs those changes, by removing the photo from your Google Drive storage (and putting it in the trash), (3) it used to be possible to sync changes in Google Drive with Google Photos, so here the photo would have been sent to the trash in Google Photos.
In other words, the same result could have been achieved by deleting the photo on Google Drive. Now, however, Google Drive can't sync with Google Photos, so what you're looking for is an app (like Backup and Sync) that syncs Google Photos with your computer. That app (or that feature in Backup and Sync) hasn't been created yet. But, I think it would be a good idea for Google to create that, also seeing as one currently cannot reliably batch upload a large folder of photos from one's computer straight to Google Photos (without creating a duplicate in Drive as well), except for using the website.