Problem :
How can I easily add text and setup font parameters to a wallpaper on Linux ?
I’ve tried Shutter but it does not permit so enlarge the text to fill the wallpaper surface.
Solution :
Gimp should work. You can install with sudo apt-get install gimp. I can’t think of any reason it would not work other than you are dealing with some weird proprietary file format in which case, just make a screenshot of the wallpaper and bring it up in gimp.