Understanding iTunes photo sync warning for iPhone

If there are previously synced photos on your iPhone they will appear in albums in a "From my Mac" folder.

You can test if a photo is a synced photo by trying to add adjustment or to delete it. For synced photos the trash bin is missing. And you will get a warning, that you cannot edit the photo or video, unless you save a copy to the library. Are there "From my Mac" albums? Use iTunes to sync your iPhone, iPad, or iPod with your computer - Apple Support

Pictures that are "owned" by Photos will not be affected. As léonie and razmee209 have pointed out, some pictures, like From My Mac pictures, are shown by Photos, but they aren't owned by Photos-- so they are treated as temporarily on the phone, and they can't be edited or deleted without using the computer, and they can't be synchronized with iCloud Photos. Pictures with "trashcans" are safe.

I have a Mac, so I don't do the iTunes transfer thing. But we have not seen crowds of people here saying they have lost pictures when they used iTunes to send pictures to their phones.

I'm pleased you have a backup! That's always the safest thing. Also, to back up your pictures you can just plug in an external drive, select the pictures, and use Share>Save to Files. Alternatively, you can backup to a computer:

Transfer photos and videos from your iPhone or iPad to your Mac or PC

Imaginita wrote:

I want to sync some photos from my computer to my phone, and I'd like some reassurance that the photos already on my phone will not be deleted.

When I try to Sync Photos via iTunes, I get the warning "Photos synced to [My Phone] from other photo libraries or folders will be removed and photos will be synced from this computer. Photos taken with [My Phone] will not be affected."

I'm pretty sure I've never synced photos from a computer to my phone before, and I don't use iCloud to store photos at all. So if I'm understanding this correctly, none of the photos currently on my phone (including screenshots, downloaded images, etc.) will be deleted, and the photos I've selected from my computer will be added, right? I have a lot of photos on my phone already, and while I have a backup of all of them, I don't even want to consider how much time it would take to re-download them all if they get deleted...

If you're getting that message that means you've synced data to your phone in the past fromm another iTunes library.