This happens when you use iCloud Photo Library with the Optimize Mac Storage preference enabled.
The Media Browser only shows photos that have the original image or a full-size preview stored on your Mac. You can make a photo temporarily available to the Media Browser by double-clicking on it. This will download the original to your Mac, where it will be available in the Media Browser until Photos optimizes it again to save disk space.
If you want to make pictures in Photos permanently available in the Media Browser, follow these steps:
- Select File > New Album... from the Photos menu bar. Give the new album a name.
- Click on the pictures you want to make available in the Media Browser.
- Click the Add button.
- Click the Albums tab at the top of Photos and find the new album you created.
- Control-click (or right-click) the album and select Download Originals to this Mac.
This will download the full-resolution version of the pictures you selected to your Mac, making them permanently available in the Media Browser.
Note: You can make all the pictures in your Photos library permanently available to the Media Browser by enabling Download Originals to this Mac in Photos preferences, but you'll lose the space-saving benefits of the Optimize Mac Storage preference.