That's why I asked you what iMovie version and operating system version you were using. iMovie 9 is not completely functional on operating systems from El Capitan and later. Black screens for still images is one of the most common complaints. It doesn't happen with video clips, only stills. No cure, really, other than to update/upgrade your software. A workaround that has worked for others is to put the still image into a Photos app slide show, export it to your desktop as a video file, and then import it into iMovie 9. Since it is now a video file it will display as a photo. You might try that if you still have iMovie 9 on your Mac.
Otherwise, you are in somewhat of a tough situation. You need to be able to upgrade to at least Mojave 10.14.6 in order to download current iMovie 10.1.14. Since you are not able to do that, you cannot get iMovie 10.1.14. If you ever had iMovie 10 on your computer before, you could update to the highest version that El Capitan can run. But that does not appear to be an option for you either.
Normally, I would say that you are out of luck, unless you want to buy iMovie 6 from Amazon or ebay as part of the iLife 6 package. However, one poster on this forum, spectre_ghost, came up with a solution that worked in his situation. You might take a look at his/her post in this thread and see if there is anything that helps you:
https://discussions.apple.com/thread/251259186
-- Rich