I know it's late in the game, but for the next guy:
Drag the group of still passed everything else in the timeline. (Select the group, shift-Z, and take them to the end.) When they're sitting on top of something else, it seems you can make them longer, but not shorter. I grabbed my group of stills and took it all the way to the end, right clicked on the group, went to duration and set the new, shorter time. Then I just moved them back to where they belong.
David Bark
Lightshine Productions
