You can lock down the timeline by holding the Ctrl key (the Cmd key on a Mac) as you add your new clip.
Steve, thanks, didn't realize I could do that. Not sure it will help me here but locking the timeline will be extremely useful. Let me be more specific and see if you have a suggestion. I have pictures on multiple layers that overlap with transitions. Basically, I need to add 2 seconds to consecutive pictures. So I don't need to add new clips, just time to exisiting ones. When I do this, all of the transitions for the remaining pictures shift and do not match up. Make sense? Any thoughts? Thanks.
The easiest way to do this is to output your slideshow (without sound) using Share/Computer/AVI.
Then import this AVI into your project (or a new project) in place of your slideshow, add your music to the audio track and use Time Stretch to speed up or slow down the slideshow to fit the length of the music.