Adding transition effects between the pictures blends them together, so your video has a smoother flow. In the View video effects menu, under Edit Movie, you'll find lots of different effects, some better than others.
You can experiment with different transitions, to find one that gives your photomontage the look you want. I like the Fade effect for its subtlety. It gives a smooth transition between pictures, but doesn't call much attention to itself.
Add the transition effects to your video by dragging and dropping them between the pictures.