It's definitely doable with paint, but as mentioned above the only filetype it handles well is BMP.

I suggest trying out Paint.NET, it's basically Paint on steroids.

To make a simple white extension (assuming that the image's background is pure white):
1 Open the image in Paint.NET (if it's a JPG, waifu2x it first to remove noise)
2 Check the color picker on the bottom left. The square in the back needs to be white (or whatever color the image's background is). If it's not, click on it, and then click on white in the color list below.
3 Go to Image->Canvas Size
4 Set the Width to something higher, preferably so the result is 16:9
5 Set the Anchor. It specifies where the original image will end up after expanding.
6 Accept, and you should be done. File->Save As, pick PNG.