iMovie 9 does not fully function on operating systems from El Capitan forward. Black screens for still images are commonly mentioned as a problem. There is no cure other than to upgrade to iMovie 10. From within iMovie 10 you can update your iMovie 9 projects and events so that they can be played and edited in iMovie 10. Unfortunately there is really no easy way to do that, since iMovie 9 is no longer sold or supported.
Do you have, or ever have had, iMovie 10 on your Mac? If so, you can use that or you can update it to highest version that Sierra can run. Here's the procedure to update:
To update/redownload iMovie 10, drag your iMovie 10 app from the Applications folder into the trash, but do not empty the trash. (You can skip that step if you had iMovie 10 on your computer before, but deleted it.) Sign in to the app store and open your purchases folder by clicking on your name at the bottom of the app store side bar. You will see your purchases displayed, including iMovie 10. Redownload/update iMovie from there. If not successful, drag your present iMovie app out of the trash (if you moved it there) and return it to the Applications folder.
If you never had iMovie 10 on your Mac, then you would need to upgrade your operating system to at least Mojave 10.14.6 in order to download current iMovie 10.
If you are unable to upgrade Sierra to Mojave, then here's a link to a thread that describes one way to get the upgrade:
https://discussions.apple.com/thread/251259186
There is a workaround that you can use. The black screen issue arises only with still images, not videos. So you can change your still image into a video. Make a one-slide slideshow in the Photos app, or iPhotos, and export it to your desktop as a movie file. Then import that file into your iMovie 9 project. It will play as a still image. You can adjust its duration by using the speed feature of iMovie. Alternatively, you can put several images into a slideshow, export it as a movie, and import the whole slideshow into the iMovie project.
-- Rich