How to delete pictures that have been imported

Hi - You don't.

There is only ever one copy of an image kept in photos - and that is always shown in the library. Albums don't have a separate copy of the image - they just reference the one in the library.

This way you can put an image in as many albums as you like and it never takes up more space.

If you want to delete the photos, that you do not want to keep, do it immediately, when you browse your photos and decide, which of them you want to add to an album as a keeper. Once you have added the "keeper" in an album or marked it as a favourite, delete the redundant, similar photo or any bad shot you one across right away.

I find the favourite heart ♡ very useful to mark keepers and to weed out bad shots or redundant photos. If we enable "View > Metadata > Favourites" we will see the ♡ as a badge overlay on the thumbnails and can see, which photos we have marked as keepers. I am deleting the redundant photos as I go, so only the keepers will be left.

Once you are satisfied with your favourite selection, you can select all Non-Keepers in a smart album with the rule "Photo > is not > Favourite".

Or try a rule "Album > Is Not > Any". However, the "Album is not any" rule has been broken in some system versions. You have to check carefully, if it working correctly on your Mac. It would be safer to mark all photos in albums as Favourites, to see, which photos are in albums. You can mark several selected photos at once.