When Apple dropped Aperture I switched to Adobe Lightroom for processing and managing photographs. One benefit is that Lightroom works with files stored in standard Finder locations rather than buried in a proprietary database. A disadvantage is that many MacOS applications, especially those from Apple, do not recognise and access images other than those from Photos, the Apple application. That includes the standard MacOS screensaver and desktop image manager.
Since I like to use my images on the Desktop, with random cycling through a selection, and in the Screensaver, I needed them in Photos. Fortunately Photos will ‘import’ images as references to files stored elsewhere in Finder without storing its own copies. As I process images in Lightroom I export them to JPG format and then import those to Photos without copying the files (easily arranged in Photos Preferences). My images are then available for use on the Desktop or in the Screensaver.
I have the Screensaver set to randomly select any or all images from Photos. For the Desktop I’m more particular so I have a smart folder in Photos that holds images with the keyword ‘Desktop’. Adding a new image to the Desktop collection is as simple as applying the ‘Desktop’ keyword to the image.
At least it was until I updated to Catalina. At that point the selection of 300+ images in my Desktop folder was reduced to about 20 of those in the folder. Thinking there may have been some issue in the update I created another smart folder with the same rule and reapplied the keyword to some images, both without success. I created another folder and added images to that with the same result. Some images, but not all, appeared in the folder once it was selected in the Desktop item in System Preferences. I tried another application, Posterino, that draws images from the Photos database with the same result.
Eventually I found the solution. Photos, at least in Catalina (I don’t recall for earlier versions), marks images it has edited with a small icon alongside others (location data, tags, reference file) in the top right corner. The images that were coming up for my Desktop all had the edit mark and those without it were not appearing.
The simplest way to add the edit mark is to open the file in Photos in edit mode (press Return) and click on the auto-enhance button. Once that is done the image is marked as edited and will appear in the smart (or other) folder in an application as it should. The right arrow key will move to the next image where the same process can be used to make it visible. Working through a smart folder with the required images is simple, if a bit time consuming.