I know how to change the settings to modify the length a of still image for new projects. But what I'm trying to do is modify an existing photo montage. I'd like to "select all", and then knock a half-second of all the still images to match the music I'm using. Can this be done?
I don't know about doing every picture at once, but I do know you can time expand the music to be longer (or shorter) than it is. It will sound funny in the timeline, but will render out fairly normal sounding, if you don't stretch too much.
If you place your cursor at the end of an audio track, then and press the control button, a 'wavy' line should appear under the cursor. You can then, while holding "ctrl" down, drag the audio track and stretch it longer. The pitch of the music will not change (unless you want it to). If used properly, you can hardly tell any difference at all.
(you could add a little time to each song, or just certain songs to get the time you need)
To shift the slides all back .5 seconds equally:
1. Select all slides on track.
2. Hit "G" for Group.
3. With all stills selected/grouped, hold CTRL, and on the right edge of the last slide, drag it to the left until you've shortened it by .5 secs. Now all the slides/stills are shortened by relative length.
As Steve mentioned, you can also speed up/slow down audio this way.