iMovie versions 8 and 9 are not completely compatible with operating systems from El Capitan forward. Black screens for stills, but not video clips, is the most commonly observed problem. No cure for it other than to update to iMovie 10. If you already have iMovie 10 on your Mac you can, from within iMovie 10, do a File/Update Projects and Events to update your iMovie 8 or 9 projects into iMovie 10 as iMovie 10 projects where they can be edited. You can still keep the older iMovie version on your computer and still access the projects from there, to the extent that they are functional.
A workaround for iMovie 8 or 9 would be to change your still clip into a video clip, and then it will play normally without black screen. To do that, you can make a one-slide slideshow with your still clip, using the slideshow feature of the Photos app, or iPhoto app, and then export the slideshow as a video file. You then import the video file into iMovie and it will play as if it were a still image. No black screen. To adjust the duration you use the speed adjustment feature of iMovie to make the slideshow clip play for a longer or shorter time like you would for any video clip. If you have several stills that you want to use in a row, just make a slideshow of them, export as a video, and then import the slideshow video into iMovie as above.
-- Rich
