The saddest phone calls are the ones about lost photos. Usually nothing can be done by then. Yet protecting against it takes fifteen minutes and afterwards it looks after itself.
Why photos on your phone are not a backup
A backup means the same thing also exists in a second place. Photos on your phone are in one place. If you lose the phone, drop it in the bath or it simply gives up, they go with it.
The same applies to photos that exist only on a computer. Disks fail, and they do it without warning.
The simplest answer: automatic upload
Your phone can send photos to online storage by itself whenever it is on Wi-Fi. You set it up once and then stop thinking about it.
Android: the Google Photos app → tap your picture at the top right → Photos settings → Backup → turn it on.
iPhone: Settings → your name → iCloud → Photos → turn on Sync this iPhone.
You get 15 GB free with Google and 5 GB with Apple. If you take a lot of photos, you will run out. More space costs around fifty crowns a month, and in my view the peace of mind is worth it.
Check that it is actually running
This is the step people skip. Turning backup on is not enough — you need to know it works.
Sign in to photos.google.com or icloud.com from a computer and see whether yesterday's photos are there. If they are, you are done. If they are not, the backup has most likely stalled on full storage.
Repeat the check every so often. A backup that is not working is worse than none, because you do not know about it.
For those who do not want photos on the internet
An entirely reasonable position. The alternative is an external drive: once a month, connect it to the computer and copy the photos folder across. Then put the drive away somewhere else, not next to the computer.
It has one drawback — you have to remember. That is why I usually do not recommend it as someone's only defence.
The rule I go by
What exists in one place does not exist. Photos you care about should be in two — on your phone and online, say, or on the computer and on an external drive.
Setting backup up, and above all checking that it really runs, is a few minutes' work remotely. It is one of those things it pays not to put off.