Hey jrzygrl2,
I understand you have a concern with 1 photo stuck uploading to iCloud Photos.
Make sure that you have enough iCloud storage available for the upload to complete. Here's how to manage your iCloud storage space: Manage your iCloud storage - Apple Support. This resource can also help increase iCloud storage space.
If you have enough storage available, try putting the Mac in safe mode to see if the upload completes. Be aware that internet access is limited in safe mode, but sometimes just starting in this mode and then restarting normally can resolve some issues: Use safe mode to isolate issues with your Mac - Apple Support
Cheers.