How to add a local image file to an image field programatically
public:// is the Public file system path that you can set in /admin/config/media/file-system.

In your case it would probably be sites/subsite.example.com/files.

To get the image you must then use, public://blog.dir/1/gallery1/image.jpg, where public:// is the same as sites/subsite.example.com/files.