So you are copying pictures from Finder folders on an external drive to the Photos app (I have to assume you mean Photos rather than the discontinued iPhoto app) on your Mac, either by dragging them in or using File>Import--is that right?
Photos is not a funnel to iCloud-- those pictures are stored in a package probably called "Photos Library.photoslibrary" in your Pictures folder. If you connect that Library to iCloud by turning it on in Photos>Settings>iCloud, then those pictures are copied to iCloud Photos. The pictures are not moved, if by that you mean "appearing in iCloud and disappearing from the Mac."
iCloud Photos is a synchronization service-- it insures that the Photos Library on your Mac has the same pictures as the Photos Libraries on your iPhone, on your iPad, and in iCloud Photos. Every device has its own local Library with all the pictures. If in Settings>iCloud you choose "Optimize Storage," then the Library can have the same pictures but they may be smaller so it saves space. But in Photos on any device you'll see the same pictures that are kept in each device's local Library.
So you have some options. I think that what I would do is copy the Library from Pictures to your external drive, and delete the Library from the computer. Then I'd double click on the external library to open it in Photos, and point Photos to use that external drive library as its System Library. You do that in Photos>Settings>General.
Then, as you add pictures to Photos the external Library will grow in a place with enough room. You will have two copies of every picture on the external drive -- one set in Finder folders, and the other set in the Photos Library (if you chose "Download Originals".) That's a backup, though it would be a better backup if the Library and Finder folders were on separate drives. See this:
Move your Photos library to save space on your Mac - Apple Support
Let us know if this makes sense or not, and how it works...
