Insufficient Disk Space while having over 100GB
I'm trying to import some pictures (about 15 GB) to photos but while doing so it keeps saying there's insufficient disk space on my Mac.
I have iCloud Photo Library turned on and theres 1.36 TB space available so that shouldn't be the problem. My own harddisk has 108.4 GB available so I don't understand why it would say there's not enough space to import 15 GB of pictures. Lately I've been importing pictures in batches (I've saved the whole structure of a SD card from my camera on an external HD, which currently has 19.33 GB available) of 15 GB. Every now and then it would say there's insufficient disk space and all I had to do was restarting my MBP and I could continue.
Optimize Mac Storage is turned on so if there's indeed not enough space, shouldn't it automatically empty some space?
Why do I keep getting this message and how can I bypass it so I can keep importing my pictures?
Addition info:
All the pictures has been uploaded to iCloud Photo Library. Does that mean it's safe to remove the folder "Photos" to clear up the space? How does it work anyway when OS X notices I'm low on space?
MacBook Pro, macOS Sierra (10.12.6)