Like in this video: 
I want to have the low-opacity background, and the effects in the background.
Hi
This can be done as follows - note you need:-
a white full screen image
the background image(s)/video(s)
the foreground images/videos
Set them out as shown in the image below
You need to adjust the transparency of the background image using the handle as highlighted in this example image.
Example movie
The rest is Keyframe animations and any other effects you need.
HTH
John
Hi,
To add to John's comments, if you need to add titles, effects, or audio, remember that Photostory only has 4 tracks available, so if you need to use additional tracks, you will have to export this effect as a video file (because you used multiple tracks to create it), then create a new project, import it and then it will only consume 1 track of the 4 you have available. If you want to do these sort of things on a regular basis, you may want to consider Movie Edit Pro as it provides you with 99 tracks that you can use to do many things.
George
Hi
Apologies - thanks to Georges answer - I realised I missed out the following from my post:-
The effect is difficult in Photostory due to the limited number of tracks.
However try downloading the trial version of MEP - and follow the procedure below (which was done in MEP to demonstrate the technique) and export the video as mp4 or mxv to import into Photostory.
HTH
John