I have so many pictures on the HD that I can't upgrade the macOS. How can I tell which pictures are on the HD (my machine's storage) and which are in the cloud?
Once that Q is done, I'll need reminder on how to move stuff to the cloud.
Pictures are not necessarily where you want to be looking to free up storage. Any picture removed from your computer will also be deleted from iCloud. You can make sure that you are using Optimized Storage, where it will save only low resolution photos on your Mac if space is needed and still have the full resolution in iCloud.
Optimize storage in Photos on Mac - Apple Support
My recommendation is to look elsewhere if you can. There are some workarounds such as turning off iCloud Sync on your device first and deleting pictures, but would never do that without a backup of those photos on external storage. There are just too many things that could go wrong.
If you don't have enough storage to do an upgrade, then you have way more problems than too many pictures. Apple recommends that you keep at least 10% of your storage free, and I've seen my apps doing crazy stuff when I had only 150GB free on a 1 TB drive. This is something that you have to get control over, and you may need to get an external hard drive to hold your extra stuff.
You didn't tell us what sort of Mac you have. Portable Macs really need lots of storage internally, because you can't walk around with a hard drive dangling from a MacAir. On the other hand, if you're stuck, you have to do something. I have a 2TB MacBook, but I carry around a small, fast, light weight, low power 2 TB SSD like this:
This is from Amazon, and it was about $120. It weighs an ounce and it's about the size of a flash drive, 50x60mm. There are others, but I would get a well-known brand. I've been bitten by off-brands that cost a silly 10% less. If you have a desktop Mac, then you can use a larger, heavier, cheaper drive.
If you don't have enough room to put your stuff, you'll be spending way too much of your time shifting stuff around in a futile attempt to get more space. "Rearranging the furniture doesn't really make the room bigger."
It could be even more confusing since you may have them in both places but different versions.
Use iCloud Photos on Mac - Apple Support
iCloud or hard drive -- where are my pictures located?
