How to add photos on desktop to iPhone 6?

I have photos already saved onto a flash drive & sent to a close friend for safe-keeping, but I also want to transfer as an alternate backup to my iPhone 6 without keeping them on iTunes. I have music/videos on iTunes but I don't want photos on the computer s iTunes - just in my photo gallery. How do I transfer?

Do you have a Mac or something else?

If you have a mac, I can help. If you have a Mac just take a USB cord and plug one end into your phone and the other into your computer. It should automatically open your photo gallery and put them in.

Otherwise, sorry I don't know.

I have a macbook pro.