How to transfer photos from PC to IPhone 6 using lightning cable?

Recently my IPhone 6 ran out of storage and so I moved all my photos over to my Windows 8 PC. I did this by connecting the iPhone to the PC using a lightning cable. An icon called IPhone 6 came up in file explorer and I clicked it. There was a folder called DCIM and then all my photos were there. I cut and paste them onto my computer. It worked and my camera roll was empty.

Then I remembered I wanted to move about 20 photos back. Connected the IPhone to the computer using a lightning cable. I copied the photos but couldn't paste them in the IPhone folder because the "paste" option was greyed out.

The photos and videos are stored in quicktime format. Does iPhone support quicktime? If not, how can I convert? Seems a bit strange considering they were originally in this format on the IPhone when I cut and pasted them across.

I can't transfer them using Mediafire Dropbox etc as it takes too long, and the quality is really bad.

I can use the lightning cable to transfer from IPhone to PC, how do I transfer from PC to IPhone?

ITunes. Or you could email them to yourself or upload them to a picture storage site & download them back onto the phone.

The way you are using is one direction. You can only copy and paste your camera roll photos and videos from iPhone to your computer, but not from your computer to iPhone.
If you would like to transfer photos from your computer to your iPhone, you can use itunes or third party software or you can email yourself your photos and save to your iPhone. If you are using Mac, I think you can also use Airdrop function to transfer photos between your iPhone and Mac.