media goes missing in iMovie

I'm using the most current OS and iMovie on a Mac mini. I am pulling video clips and still for a Photos album. I close my work and when I come back to it, most of the files are missing. My guess is that since I have a very large Photos library and my originals are being uploaded to iCloud and moved off my Mac. So only the thumbnail is there when I reopen iMovie. That's a guess. I can solve this problem by exporting my stills and clips to a folder on my Mac and pulling from that folder, but it is a lot of work.

Is there a way to download and keep full size files from one Photos directory? Or can I make iMovie import the media to my project file so it won't disappear?