Hi,
What versions of iMovie and operating system are you using?
As I understand it, you have placed a photo, not a video, in the iMovie 10 timeline and a song in the audio well beneath the photo. When you play the song the photo gradually fades out as the song progresses. Is that correct? If you followed a different procedure in setting up your project, would you describe the steps you did? Also, could you send a screen shot of what you are seeing?
The picture should not be fading out unless a fade has manually been applied to it.
-- Rich