I don’t think there’s a way to do that via the security settings any longer… but you can change the settings for each album, so at least you don’t have to go photo-by-photo. Go to Photos > Albums. Don’t open any of the albums. To the right of the number of photos in the album, there’s an icon indicating the security for that album. You can click that to change it. Since photos you post individually on your timeline are all in an album called Timeline, you can change those all at once.