I'm having the same problem as Keithj999.
Your solution is not viable, as it only pertains to Mac computers, and this is the PC thread.
The problem: We have a PC computer. We have a lot of photos on the computer which need to be uploaded into the iCloud library. The only way this seems to be able to be done is via the iCloud website. When going through the iCloud website to upload into the iCloud Library (NOT photostream, as my understanding is everything there deletes after 30 days..) from a PC computer, you can only select one photo at a time to be uploaded. Having thousands of photos, this clearly is not a good solution to the problem..
The desired outcome to the problem: To get thousands of photos from a PC computer into the iCloud Library in an efficient manner.
The solution for the desired outcome: ???
I've been looking for a few hours now on how this might be achievable, and I have yet to find a solution.