iMovie cannot see photos that optimized and stored in iCloud

. Sometimes it can see some random photos, that for some reason OS optimization mechanism left over, but you will have troubles to get your recent photos.

The only way to import needed photos is by export feature(File -> Export) in Photos app.

If you want o use photos from an optimized iCloud Photo Library in the Media Browser, make sure you download them before you try to access them from other applications.

First select the photo in Photos and enlarge it. That will force Photos to download it from iCloud. Only then try to use it in iMovie or other applications.

You don't have to download all originals, just the ones you want to use for our current projects.

Keep "Download originals" disabled - just force the download for the photo you want to use by enlarging it in Photos.

I believe the most straight forward option is to use Export feature. By using it, I assume, you won't increase the size of your local library

temporarily the size of the library will increase, just like when enlarging or editing a photo. After all, the photo needs to be downloaded from iCloud to export it. Photos will keep the photos you use in your library for a while, to avoid having to download it over and over again. There will always be a few local copies, that will be discarded again, when local storage is needed.

It's not a bug but how it works. In order to use one of the photos in another app or in a project in Photos the full sized version must be downloaded first.