I can't duplicate that issue here, but apparently it is (or was) common to have still images show up as black or green screens or lines. I found various suggestions online -- no one reported a definitive cause or solution, but people offered suggestions that worked for them. Since I can't duplicate the problem I can't vouch for any of those suggestions, but they include..
- It has to do with image size -- reduce the dimensions of the image (i.e. 1000x500 to 750x325)
- It has to do with file type -- try converting file type (i.e. JPEG to PNG)
- It has to do with the video card in the Mac
- It has to do with the version of iMovie
- Try deleting & re-importing the images
Of course you could also try the usual troubleshooting steps..
- Hard shutdown/restart
- Try the same images in a different iMovie file
- Try the same images in a new (fresh) iMovie file
- Try the same images in iMovie under a different User
- Try the same images in iMovie on a different computer
- Try different images and see if the problem affects all images or only certain ones
- Try reinstalling iMovie
I've attached one of our basic iMovie courses -- maybe something in there will help, or perhaps another iMovie user has encountered this and will be able to offer a suggestion..