Why my iOS free space didn't increase after "empty trash"
In iOS8, deleted photos will remain in iOS photo library for 30 days before permanently deleted - this is automatic and you don't have to do anything. In case you need to immediately reclaim the space, you can also use the following steps to permanently delete them immediately.
launch your photos app, and select "Recently Deleted" album
Click "Select" on the top, then "Delete All" at the bottom.