Hey there, Christoph,
Thanks for stopping by the Apple Support Communities. If I understand, you'd like to upload pictures that are on an external drive to iCloud Photos. I'm happy to help!
It sounds like you are trying to upload using the iCloud.com website. To do this, your pictures must be in a folder on your external drive, as iCloud.com cannot open and browse your Photos app or Photos library directly in this manner. You can use the steps in Export photos, videos, and slideshows from Photos on Mac - Apple Support to export your photos to a designated folder on your external drive.
Alternatively, you can set iCloud Photos to automatically sync your pictures through the Photos app. You can do this even if your Photos library itself is on an external drive (you just need to make sure the external drive is connected in order for syncing to occur).
To do this, first use Move your Photos library to save space on your Mac - Apple Support to get steps on moving your Photos library to your external drive. Then, designate that as your System Library: Designate a System Photo Library in Photos - Apple Support. Once you've done that, you can set up and use iCloud Photos in the Photos app on your MacBook, and your pictures will be accessible in the Photos app and iCloud Photos, but stored on your external drive: Set up and use iCloud Photos - Apple Support.
More information on using the Photos app in macOS can be found here: Use Photos on your Mac - Apple Support.
If this doesn't address your issue, don't hesitate to respond back. If you do, please give a few more details on your situation, including the answers to the following:
- Are the pictures in the native macOS Photos app, or are you using a third party app?
- If you're using the macOS Photos app, have you moved the native macOS Photos app itself to the external drive?
- If you're using a third party photos app, are the pictures in a separate folders on your external drive, or are they saved within that app?
Best!