why have my photos become videos?

It sounds as if you have activated Live Photos. This is a feature of 3D Touch on the iPhone 6s series of phones. When you open the camera app, in the top bar of the phone in the center, you will see a circle with a dotted line circle around it. That turns Live Photos off/on. If it is white, it is off, yellow it is off. That is what is making the 2-3 second video clips. Turn that off, and you will be okay.

Depending on your computer, when you import these photos to the computer, you will see both the initial photo, and then a short video clip with the same name.

To make a still photo from a Live Photo:

When viewing the Live Photo on your iPhone: tap the share button, tap the duplicate button, then tap Duplicate as Still Photo.

To turn the feature off, tap on the Live Photo icon in the camera app:

(Yellow is on).

you may have inadvertently slid the photo selection to video

you may be able to extract them with this

https://itunes.apple.com/us/app/vid2pic-video-to-picture-converter/id930371755?m t=8

Those are called live photos. Read this.

https://help.apple.com/iphone/9/#/iph1e767793c

Read more about it here.

http://www.apple.com/iphone-6s/cameras/

You're welcome! 🙂

You're welcome. I can get the still when I use my Windows computer. Glad I could help, at least enough to identify why it was happening.