Pictures folder on iMac has a 4 GB Photo Library In my Home directory on my iMac I have a Pictures folder. In it is a 4 GB Photo Library named Photos Library.photoslibrary. I have iCloud Photo Library turned on, and it has been on for more than a year.

Why do I have this 4 GB photo library on my iMac? Shouldn't all photos be in iCloud? Can I delete it or should I just leave it?
Leave it. Photos still needs to download your Photos when you decide to view them. It will either download a device-optimized or full-resolution copy depending on your chosen settings.

Think of this file as a local cache of your photo library, although it also contains other things that are not stored in iCloud, such as Smart Albums and facial recognition data. If you delete this file Photos will need to recreate it and rebuild your library from iCloud data. In particular, it will need to re-download thumbnails for all of your photos.

The idea is that if you do run low on storage, Photos will start purging photos you haven’t viewed in a while, in albums you haven’t specifically marked as Download Originals to this Mac.