In the photos app; select an album ("camera roll" for example), press "select" at the top right corner of the screen, tap as many pictures you want to delete, press the trash can icon in the bottom right corner of the screen, confirm the action, return to the "albums" selection screen, select "recently deleted" album, press "select" again, select the photos again, press "delete in the bottom left corner, confirm the deletion. Note: this makes the photos non-restorable. Otherwise, the pictures stay in the "album" for 30 days before final deletion.