As sberman suggests, the Photos app is an ideal way to organize all your pictures-- that's what it was made for!
With Photos, pictures are stored in a Photos Library. On a Mac, the Photos Library is found in the Pictures Folder. iPhones and iPads also have Photos Libraries that store their pictures. iCloud also has a Photos Library.
iCloud Photos is used to synchronize all the Libraries on all your devices. (For that, all the devices must have the same Apple ID and must have iCloud Photos turned on.)
So, if you set up iCloud on your Mac, and if you use the Photos menu "File>Import" to collect all your pictures from various places on the Mac, then they will be copied to your Mac's Photos Library. And then all those images will be copied though the internet to the iCloud Photos Library. And all the pictures that were at iCloud Photos but were not on your Mac will be copied from iCloud Photos to your Mac. You'll have them all right there. And, if your Mac gets destroyed in a tornado (there seem to be lots, lately) all your pictures will still be available from iCloud Photos.
And all this happens with your iPhone and iPad and iWhatevers, and all the pictures will be everywhere. If you take a picture with your iPhone, it will automatically be added to iCloud Photos, and then to your Mac. And this works for both your Macs-- so all the pictures will be on both Macs.
There may be a problem with storage. On any device, in the Photos iCloud settings, you can set it to "Optimize Storage" by keeping only smaller versions of images. This relies on iCloud to keep the full sized ones. Most of us use at least one Mac to "Download Originals," so that we have full copies for backups. This may require an external drive if you have tens of thousands of pictures.
You can use the link that sberman provided to learn more, and experienced folks who use Photos are always here to answer questions.
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