my hard drive is almost full of photos but I thought when I upgraded to Yosemite and bought storage they all went to the cloud

A copy of your photos may have gone to iCloud but the photos stay on your Mac. Apple does not offer any cloud storage option where the only copy is on iCloud.

You can reduce the storage space required on your Mac for iCloud Photo Library by opening the Photos app and clicking Preferences on the menu name Photos. In the Preferences window select "Optimize Mac Storage."

You need to maintain a minimum of 10 - 15 GB of empty hard drive space (more is better!) in order for the OS to function properly, so if you have that many photos, I'd suggest moving them onto an external hard drive to gain some space on your hard drive.