The new Photos app for Mac allows you to store all of your photos on Apple's servers as part of their iCloud service. This makes your entire Photos library available on all of your Apple devices including Macs, iPads and iPhones. You can add, edit or delete photos on any device and see the changes on all of them. You can choose to store original photo files on a device, or have the device only use optimized versions of the photos. You can also view your Photo Library on iCloud.com.
When you first run the new Photos app it will ask you if you want to convert your iPhoto Library to a Photos Library and then it will ask you if you want to store everything in iCloud.
Now if you answered no to that second question you can always change your mind and go to iCloud in System Preferences. You would need to turn on Photos, of course, which you may already have. iCloud Photo Library is what you need to turn on to store all of your photos, your entire library in iCloud.
When you do you are going to be prompted, telling you what this involves, and that you may need to upgrade your storage base. If you only have a few photos then, of course, it will stay within the 5GB of range that you get for free from Apple. If you go more than that then it is going to put you into one of the priced here. The cheapest one being only a buck a month. I've got 20,000 photos in my iPhoto Library. I take a lot of photos on vacations and I've been doing it, well, for at least fifteen years now. So I've got plenty to go to the next level which will end up costing me more.
As an example here I've got my iPhone here on the right and I'm about to take a picture with that. I'm logged into the same iCloud account with this iPhone and with my Mac which you see on the rest of the screen. So I take this picture and it should upload it now to iCloud. Once it is uploaded to iCloud then it will appear here in my photos collection on my Mac as well. So there we go. We see it appear right here, scroll up and there is the photo I just took.
Now likewise if I were to select this photo and then delete it, it will give you a warning here that this photo will be deleted from all devices. In other words I am deleting it from my, computer, from iCloud, and then that is going to propagate to all my different devices. So it will now be gone from everywhere.
The same is true when you edit a photo. The edits will then appear on all your different devices. So you can edit a photo on your Mac and then see the edited result on your iPhone and vice versa. You always have the original photo saved in iCloud so you can always revert to the original photo even on another device.
Now you also have the ability when you have deleted a photo, if you've made a mistake, you can go to File and Show Recently Deleted to see all the recently deleted ones and restore one that you may have accidentally gotten rid of.
In addition to seeing your photos in the Photos app on your devices you can also see it if you go on the web to iCloud.com and you go into the photos app in there. There you can see all of the photos just as you would anywhere else. The great thing about this is, of course, you can also log into this via a non-Mac or iOS device. So you can log into iCloud.com on a PC if you have one or if you are using one temporarily and have access to your photos. There is even a lot of things you can do here. Select photos, download them, email, and delete photos. So you can manage your photo collection from the web, not as good as you can do it from the app, but you do have a lot of the functionality.
Now a question I get a lot is if all of your photos are in iCloud do you need to have copies of them on all your different devices? So if you have a very large library does it have to be copied everywhere?
Well in Photos is you go to Photos Preferences you've a control for that. You go to iCloud and you've got iCloud Photo Library turned on. You have two options here. One is to have the originals downloaded to this device. The other is to optimize the storage. Since this is my main Mac, my desktop Mac, I have all the originals here. So I have a complete copy of all my photos in the Photo Library here.
However on my iOS devices and my MacBook I have Optimize Mac Storage. This means that I won't get the full resolution photos unless I need them, say for editing, and it is going to manage the space. So if I'm out of space on one of these drives it is not going to have all of the photos there unless I actually want to look at a specific one. I'm not exactly sure what happens if you are really short on space like you have a 60G photo library and you have 3G left on your drive, what exactly you will see. But this will save you from having a duplicate of that 60G library on every single device.
This is what you want to have turned on everywhere except your main computer, your main Mac, and then you can use your main Mac as a backup basically of what's in iCloud because you have all the originals stored in that Photo Library file that is in your pictures folder.
Now if you want to use photos in a non-conventional way you still have a lot of options. For instance, I'm going to quit Photos here and I'm going to relaunch it but holding the Option key down. You can see it lets me choose from different photo libraries that it finds including iPhoto libraries. I can create a new one as well.
Notice one thing here. This Photo Library is called System Photo Library. That is the only library that will sync with iCloud. There can only be one that syncs with iCloud across all your different devices especially true since iOS doesn't have the ability to select from multiple libraries. There is only one.
However, I can create a new Photos Library say for a temporary project that maybe I'm involved with. Say I'm in school and I'm doing an art project and I need to take hundreds of photos and I don't want them to be mixed with my family and vacation photos. I can create a new library and store them there. Every time I run iPhoto I can hold down the Option key to bring up this menu here and choose which one I want.
Now keep in mind whichever library you have selected is going to be the one that photos are going to import in to. So if you are doing something like a special project you want to make sure that you are using your System Photo Library mainly, switch to one and then switch back to the System Photo Library when you are done so that you don't make a mistake the next time you import photos.
So one more thing I want to show you. If you go into the Finder and you look in your Pictures folder you will find your Photos Library. You can put that anywhere you want. This is the main place most people will have it. But say if you are in a situation where you have a MacPro and you have a large external drive for a huge photo collection. You can move this to the external drive. You will do that with Photos closed, quit, and then you will open up Photos and locate the Photos Library again. But iCloud doesn't care exactly where the System Photos Library is. It doesn't need to be in the Pictures folder. So you do have that flexibility to be able to move things around.
Also Photos has the ability to import photos and leave the photos where they are. Not import them into the Library. If you use that, those photos will not sync with iCloud. They need to be in this library. Most people will use it normally, that way.
I'm going to include some links to some Apple frequently asked question pages and things if you've got more questions about how iCloud Photos Library works.
Check out Apple’s iCloud Photos Library FAQ.
Check out Apple’s general information page about iCloud Photos.