Premiere Pro really does make this so easy.
First off make sure you've got enough video tracks. To add more, right click on the area to the left of the track and select add.
Now add your video to track 1, then you stills in the layers above this. I inported a few JPG images, dragged them to the timeline, then extended the duration of each image to the length of the video clip in track one (right click on the clip and select duration.
At this stage, all your clips will be on top of each other and probably too large. So select the clip in the top most track (click on it so that it goes darker). Now move to the effects control and expand the motion effect. Adjust the scale and you'll see the still change in size (and expose the still underneath it). Now move the position settings to position the clip just inside the screen. Repeat this for the other clips. If you've got more clips than fill the screen, don't worry, we'll address this later.
So now you should have something like this:
Next we need to start scrolling the stills. So select the clip in the uppermost track again. Go to the motion effects control and click on the stop watch icon to create an animation. Now move timeline marker to where you want the clip to be at the top. click the round icon next to the position settings to add a keyframe (it will say Add/Remove keyframe if you hover over it). Now click on the created key frame and move the position markers so that the still is at the top of the screen. Remember the difference between the start position and the end postion!
Now move the other clips in relation (i.e. by the same amount). In my case it was 706. This ensures the same gap is maintained. And that, as they say, is that.
Apologies if this isn't a very good explanation, but I had very limited time to create it!!!
(you'll notice that in my hurry, I didn't keep a constant gap as the clips weren't the same size. Doh!)
http://www.marcpeters.co.uk/movin-still.wmv 
OVERLAYS & PICTURE IN A PICTURE 
Linear Mode
