How do I convert any image format (jpg, png, etc.) to wallpaper?

On some of my virtual desktops I have images which I found on the net, downloaded, converted from jpg to wallpaper format, and then used them as wallpaper for specific virtual desktops.

Unfortunately I don't remember how I did this. It was a couple of years ago, but it seems reasonable that it should still be possible to do this. After all, I still use the same system: Debian 8.11 Jessie, with Plasma SC Version 4.14.2.

I am very grateful for all help.

Thank you.
Eveanne