First of all, I think it is always good to follow directions to update a program whenever you get that message. It will keep your computer running faster and keep you up to date. One of my clients had an iPhone with the new Photos program and still had iPhoto on her iMac because she had not updated to Yosemite. That was confusing. It is a bit clearer when you are dealing with the same program both places.
We, older people, have an outdated way of thinking about photos. Computers and digital photo taking has totally changed how it all works. It used to be that you had film with your originals and paper copies of your photos. You could hold them in your hand. You could lose them or they could get mouldy in the basement. You could put them into albums or boxes. You could pass them on to your children.
Now, they seem sort of ephemeral. Where are they really? How can I organize them and keep from losing them? It is so easy now to take pictures that we all have SO many of everything and often there are multiples or pictures that we don't want to keep, but who has time to go through them all?
Use the 3, 2, 1 principle of saving pictures and videos that you really want to keep. Have 3 copies saved in two places and put one in a safe deposit box or offsite somewhere that you can access and update once a year, or on some regular basis.
Three copies of any picture that you really do not want to lose should be saved. One is your primary and two are back ups.
Two copies should be on different media. Do you remember floppy disks? Most computers today do not have floppy disk drives, so you cannot access them any more. The CD drive is going the way of the floppy drive, so it is possible that if all of your copies are saved ONLY on CD you may not be able to access them easily later. So, you could have one copy saved on a hard drive and one on a CD. Or you could save one on your computer and one in the cloud. I think I might keep one on my computer, but have two external drive backups. One of them would be my regular back up that I keep at home and one ONLY has photos and is kept in a safe deposit box at the bank or something.
One is kept offsite somewhere. If there is a disaster at your house and you lose your computer and your backup at least you will have a copy of everything that you last backed up in another location.
http://www.dpbestflow.org/backup/backup-overview#321 tells more about this and explains it as a way that ALL important files should be backed up. DP stands for Digital Photograpy and this comes from the American Society of Media Photographers, so it may be overkill for us home picture takers, but it is helpful information.
Like I said above you should always keep all of your devices up to date so that they are using the same software, especially if you want to sync between devices.
The new software called Photos does something interesting. It will hold up to 1000 photos and as you add new photos after that it drops off the oldest ones and only keeps the newest. When a photo is deleted from your phone it is deleted from all devices that are syncing to it. You can turn on iCloud storage if you want to keep photos. This will give you 5G of storage space for free. If you want more you can pay for it
What if you want to store your images forever (or as long as possible)? On the Mac help page it says:
iCloud Photo Library stores all of your original photos and videos in iCloud, but we always recommend you keep back up copies of your Library. You can download your photos and videos from iCloud to your computer and store them as a separate library, transfer them to your computer with iTunes, or store them on a separate drive.You can go to http://icloud.com to see what photos are stored there. If you want to do something with those photos you can click on them. Once a picture is open you can share, delete or download it. If you want to do this to multiple pictures choose the word SELECT at the top right of the screen and you will be able to put a checkmark in the ones that you want to act on.
If you don't want to spend more money on photo storage there are lots of options. You can download them to your computer and save them in the Pictures file. One thing lacking in the Pictures file is automatic organization. You have to put pictures in folders and organize them yourself. You can then save them to an external hard drive, or two. You can upload them to an online storage space. Amazon Prime offers unlimited storage space for free and so do many other places. See information on Picasa and Flickr below.
There are many programs that store photos. The one that comes with your PC is called Windows Photo Gallery. Photo Gallery can find all of the pictures in your Pictures file and automatically store them in folders by date. You can then go in and do all sorts of things with your pictures. You can reorganize them into different files, edit them, give them tags or keywords so that they are easy to find.
There are other programs online and that can be downloaded to your computer that do similar things. Picasa is owned by Google and now works with Google Photos. It has an online feature and you can download a program for your computer that will sync with it. It gives you the ability to organize and edit your photos. Shutterfly, SmugMug, Flickr (which is owned by Yahoo and requires a Yahoo login).
Helpful article on Photo Storage
So, what am I going to do?...... Someday when I have lots of time I am going to organize all my photos. I think I will get two hard drives. On both of them I will upload ALL of my photos. On my computer and my phone (and iPad) I will have the same set of the most current photos in the photo program so that I can look at them and easily use them. Then once a year... September?, January?, my birthday? I am going to get the second hard drive from the safe deposit box (or where ever I decide to save it) and update BOTH hard drives and keep just the current year on my computer, phone, and iPad. Someday..... but then I still have VCR tapes waiting to be converted and cassette tapes that need conversion.