Hi, Oscar,
First, upgrade to Mojave and see if that solves it. If it doesn't solve it, see below.
Sometimes photos that are too high resolution can cause problems.
This might not work but here is something you can try. Using a pink photo to test, try lowering the resolution of the photo, so that the long dimension number is less than 4000. So, if the photo is 4200 x 3200' reduce the 4200 number to, say, 2500, with the 3200 number scaled proportionally. You can use the Preview app on your Mac to do this. Put the photo on your desk top and open it in Preview. Then click on Tools/Adjust Size. A dialogue box will open and you will see a box that will let you make the adjustment. After reducing the resolution, save and drag it into iMovie and see if you can edit it.
If still.no luck you might try converting the photos to jpeg, if they aren't already.
-- Rich