The easiest would have been to set the desired Duration, prior to Import, via Edit>Preferences>General, but that will not help you now.
For a change in the Duration of the stills, assuming that you will want everything to follow along with your changes, Click on the Tail of each image in the Timeline and drag to the left. The cursor will turn into a big ] with a double arrow. Open the Info Panel (Window>Info) and watch the Duration change to what you want. I would expand the Timeline horizontally to have fine adjustments. Repeat.