Google Photos can't free space when pictures are on SD card
The SD card on Android is by default mounted in read-only mode: Google Photos can copy photos from the SD card to the server but it cannot edit, create or delete files on the SD card.

To allow Google Photos access read-write to the SD card, follow these steps:


Open Google Photos > Side navigation drawer > Device folders > Open a folder that is located on the SD card
Try deleting an image in that folder by long pressing on it and then pressing the bin icon in the toolbar.
A prompt titled "Give Google Photos access to your SD card?" - allow it.
A prompt with a message saying "Grant Photos access to your data, including photos and videos, on ?" - allow it.
The photo will be deleted.


This method was tested on my Samsung Galaxy Tab A.