restore a single Photos image from Time Machine backup

The image files are inside the backed up library package. As lang as you are starting the search from outside the Photos Library package, the search will not show the image files inside the package.

If you are afraid to overwrite your Photos Library when you restore it from a backup, try to drag a copy of of the Photos Library from your Time Machine backup to a different folder.

Ctrl-click the copy of the library, then use the command "Show package contents" and open the folder "originals" in the window that opens. Start your search for the filename of the photo you want to recover from this folder. In Photos 4 the originals folder is organized according to the date of import, with subfolders by year, month, day of import. Try to find your missing photo this way.

Restore the Photos Library to a different drive, so you do not overwrite your current Photos Library.

You will need to be able to run the Photos.app to find the photos inside the saved Photos Library. And you cannot run Photos on a library that is a part of the Time Machine backup.

Is the system version you have given below your question still valid? In the older system versions up to macOS 10.14 it has been possible to find a photo in the Library package by searching for the filename, but this is no longer possible in the more recent system versions, because Photos is no longer storing the original image files under there original file name but is renaming them. This is making it very hard to recover photos from the backup of a library, without opening this backup in Photos.