Building the slideshow in iPhoto is probably the best answer . Second best is to create the transition settings you want to use, then apply the transition between the first two photos. Now select the transition itself in the timeline and hit Command-C to copy it.

Select the next photo, hit Command-V. Next photo, Command-V. Next photo, Command-V. Repeat until done.

Still not the greatest, but it beats using the Transition panel repeatedly.

-rob.

In iMovie HD 6: If you have all the photos showing as clips in your timeline, highlight the first clip, then go to the last clip, holding the shift key down and click. This will highlight all the clips. Now select editing->transitions and choose "cross dissolve" (or whatever), set the speed, click on "add" and the transition will be inserted between each clip.