Transferring photos: can't delete originals

[WestofYouLB]

when I transfer my photos, the (Windows) program we use reports it can't delete the transferred photos on my iPhone 6+ because they are all write-protected.

I now have far too many pictures on my phone, so I need a solution, both for deleting the ones remaining on my phone and for the future so this doesn't come up any more. In Windows/DOS of course I can change protection, but have no idea how to do it in an Apple device. Or prevent recurrence.

Thanks.

 

[RedOrchestra]

Did you originally sync your photos to your iPhone using a different PC?

 

[WestofYouLB]

Did you originally sync your photos to your iPhone using a different PC?

I don't remember syncing it at all. It would have been years ago with my 4. But the same PC, if I did.