Deleted imported pictures disappear from my albums

That is to be expected. Albums are not storing copies of your photos. Albums are just referencing the photos in your library, just like an index. If you delete a photo from your Photos Library, it will disappear from all albums you have added it to.

Generally, Photos is storing any item you import only once, and all albums you create or the predefined albums created by Photos are referencing the one copy of the photo in your library. Do not delete any photo from Imports or all Photos, or any album automatically created by Photos, unless you want it gone everywhere, completely.

If you are deleting photos from Imports, so you can see, which Photos have been already added to an album, try a smart album instead of deleting:

File > New Smart Album:

This smart album will only show you the photos not yet in any album.

My screenshot is from macOS 11 Big Sur; is the system version given below your question (macOS 10.13 ) still valid? I am not sure, if this smart album already worked on High Sierra.

In the early versions of Photos we could write "Album is not %"

Read the following, particularly Note d, regarding deleting photos from a library:

Deleting Photos from a Photos Library

1 - regardless of where you are in the library, i.e. Photos, Memories, Favorites, People, Recents, Imports, albums or smart albums, select the photo(s) you want to delete and use the key combination of Command+Delete to move the photos to the Recently Deleted smart album.    Without the Command key held down you'll be presented with a confirmation Yes/No window regarding the deletion. 

2 - click on the Recently Deleted smart albums in the sidebar.

NOTE 1: If you're using Photos 1.2 or earlier use the File  ➙ Show Recently Deleted menu option.

3 - in the Recently Deleted album you can opt to let the photo automatically delete after the 30 day waiting period is up, delete them immediately or restore them to the library.

NOTE 2: deleting a photo from an album, slideshow, book, etc., with the Delete key only deletes the photo from that item. There is no key combination that will delete a photo from a project and move it directly to the Recently Deleted smart album.

NOTE 3: deleting a photo from a Moment or Collection (the Photos window) or the All Photos window deletes ALL occurences of that photo in the library.