You cannot set the duration for all the photos after they are in the timeline. You need to lengthen each photo by the amount you want using the trim handle on the right.
You need to set the duration for the photos before putting them on the timeline. Open the first photo,in properties tab set the duration you want, press set default. Add the photo to the playlist. All subsequent photos will use the default duration.
This will work to insert a new photo.
Ripple must be on. 