I want to overlap a photo onto another - this is simply to extend the borders of the original picture on all sides so that it fits on a website - at present it's "cropped" with a black border because it doesn't fill the full frame.

If I use a white background then there's no problem but I want to use a black background!

I can't simply enlarge the picture as this distorts the image.

Is there free software that i can use to extend the borders of my pics - I'm not a techno geek so has to be basic. Ideally a software where I can simply drag a duplicate across the original and save the the pic - something simple and stress free? TIA