back up/icloud
are my photos on iCloud and if so do I need an external back up also?
iMac 21.5″, macOS 10.15
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They would be on iCloud if you have iCloud Photos turned on in your Mac. See: Set up and use iCloud Photos - Apple Support
I use iCloud Photos on all of my Apple devices but I also back up my Mac locally. Note that if you have iCloud Photos turned on and also have the Optimize Storage option turned on in Photos and if your Mac is low on storage space, some of the photos on your Mac may be smaller optimized versions and a local backup won't be able to backup the originals.
Whether or not you "need" a local backup depends on your comfort level. I sleep better having one (while I also appreciate the ability of iCloud Photos to sync my devices and to keep an offsite copy of my photos). Note that with iCloud Photos, if you delete a photo accidentally or otherwise, you only have a limited time period (roughly 30 days) to notice and to recover.