In Photoshop if you want to stretch some part of an image you can use the marquee tool to select the pixels you want, use Ctrl+J to duplicate and then Ctrl+T to stretch it.
I have a video recorded using a mobile phone in the vertical position and I want to do the same thing to repeat the sides of the video (it's a person talking and there's a painted wall behind him). Is it possible?
It is possible, but it might look a bit weird. Video is constantly changing - even just a video of a wall will have noise in it, so static images tend to look a bit odd in comparison. But Ii you stretch one pixel of the noisy moving image, you'll get lines of noise moving either side of your image, which might not look great either.
However, if you want to give it a go, duplicate your original image, use a mask to only show the edge that you want to extend, then adjust the scale in one dimension only (you'll need to click on the little link icon to only adjust the x-axis scale).
Alternatively, What many people do for a backdrop is to copy the image, scale it up to fill the screen, then blur, and darken it, and place it behind the original image.
