Hi,
You can access the Photos app from iMovie by clicking on the photos item in the iMovie media sidebar.
However, iMovie will not recognize the Photos app unless the Photos app library is set up as the system photo library. To do that, open the Photos app and in the menu click on Photos/Preferences. In the box that appears, click on the box that says "use as system library". If it is grayed out, then the Photos library is already your system library and you need not do anything further.
Now close and then reopen iMovie. Open a project in iMovie. You should see a Photos item listed in the side bar. Click on that and you will have access to all of the photos in your Photos library for use in iMovie.
Another way to import photos from your Photos app library into iMovie is to open the Photos app and select File/Export. You will get a box giving you the option to export the Original of the photo or a Modified version if you have made modifications.. Once you make your selection and click Export you will get a box where you can select your destination, e.g, Desktop. Using the export function allows the exported photo to retain its name and EXIF data.
You can also drag a photo from the Photos app into iMovie, but you can't do it directly. You must first drag the photo to your desktop and from there you can drag it into iMovie.
-- Rich