how do I keep only thumbnails on Mac and full photo file on iCloud

I am using "Photos" in Yosemite 10.10.04. The sum of all my photos is too large for my Apple Air's smallish storage. I would like to store thumbnails on the mac air and keep the original files on iCloud. Is this possible and/or practical?

Or

Should I continue to use an external drive for the original files instead of the iCloud? (This is what I did before I switched from iPhoto to Photos.)

What is the best process to accomplish this?

I've tried moving the photos to iCloud previously. It seriously hung up the machine and I had to run a recovery. I gave up, and I am waiting for some clear instructions from Apple for 10.10.04 Photos and storage of same on iCloud. I am somewhat fearful of trying to import so many files to either the external of to the cloud without some better information.

Can anyone help me with this longwinded question?