This happens when you use iCloud Photos with the Optimize Mac Storage preference enabled.
The Media Browser shows only photos that have the original image or a full-size preview stored on your Mac. To make a photo temporarily available to the Media Browser, double-click it. This downloads the original to your Mac, where it's 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:
How To: Show the User Library Folder in macOS Sierra Chris Hauk - Dec 30, 2016 While many Mac users may never need to access their User Library folder while running macOS Sierra — some advanced users, may need to get in there and access support data and such when troubleshooting. 2014-9-1 If you don't have the left-hand sidebar showing then you can enable it via option-command-S on a Mac, control-S on a PC. If Books doesn't show as an option under your Library then go into Edit Preferences (on a PC Alt-E should get the Edit drop-down menu to appear) and on the General tab make sure that Books is ticked. 2020-4-5 Options to Show User Library. Click on your Users folder, open it in Column view, and select View Show View Options then check the box for Show Library Folder If Library isn’t an option, check that you’re in Column View; Select your User folder, and press Command+ J. Then check the box for Show Library Folder. Ever since Lion (OS X 10.7), Apple has hidden your personal Library folder (/Library) by default. In Lion and Mountain Lion (OS X 10.8), you could make the folder visible, but it required some work. 2020-4-6 How to view a Mac's /Library folder. There are three ways to easily view your hidden /Library/ folder. In macOS Sierra Apple added a Finder keyboard shortcut that. 2020-4-3 By default, the /Library folder is hidden on Mac. This is a folder that Apple feels you shouldn’t need access to most of the the time, but occasionally, you may need it for specific things. I use it often to view the preferences for the apps I have installed on my Mac.
- Select File > New Album from the Photos menu bar. Give the new album a name.
- Control-click (or right-click) the pictures that you want to make available in the Media Browser. Choose 'Add to [new album].' Or drag the pictures to the new album.
- Find the new album you created.
- Control-click (or right-click) the album and select Download Originals to this Mac.
Mac Library Updates
This downloads the full-resolution version of the pictures you selected to your Mac and makes them permanently available in the Media Browser.
Mac Library Caches
If you want to make all the pictures in your Photos library permanently available to the Media Browser, you can select Download Originals to this Mac in the iCloud tab in Photos preferences. When you do this, you'll lose the space-saving benefits of the Optimize Mac Storage preference.