macOS 10.12.6 running on 4 GB intel core duo. Have new iPad Air if intermediatery step would be to download to iPad and then to Cloud.
iMac, macOS 10.12
For as far as i know you can just login to iCloud web on your old iMac, make an iCloud folder in the iCloud Web app and upload the photos with no problem. I did it that way when migrating from a 2009 iMac 3 years ago
Are these 15000 pictures in the Photos Library, shown and edited with the Photos app? Or are these pictures in Finder Folders somewhere on your iMac?
So then the question is what do you mean by "download to… Cloud?" The normal place for pictures is in Photos on the Mac copied to iCloud Photos at iCloud.com. Another option is to have pictures in Finder Folders in iCloud Drive where they will be copied to Drive at iCloud.com.
Finder in iCloud Drive is uses a File Management System. Photos and iCloud Photos use an Image Management System.
A File Management system like Finder relies on filenames, file created date, file modified dates, file extensions, and so on. In Finder you can sort files in folders using those things, for instance, rather than content, because the content in different files is so different. None of that is useful in Image Management. Pictures have similar content, and so their management depends on Capture Dates, Titles, Captions, Faces, Keywords, Objects, Locations, text, and things that are parts of pictures but that aren't in files at all-- and there are lots of organizing options in Photos using those image attributes. Since pictures aren't necessarily in files, then sorting pictures by file date or file name doesn't make sense.
So, what do you mean by "download to… Cloud?
Uploading 15,000 photos from 2009 IMac to iCloud?
