Getting a non-standard size photo printed
I have a photo I took that is in 9:16 ratio that I want to have printed up and I don't want to crop it to fit, say, a 16 X 20 standard print size. If I have it blown up to 20" wide, the height would come out to 11-1/4" high. What I want to do is add a 4-3/4" band to the top or bottom so that it has a 16 X 20 format and I can get it printed cheap. Then I can trim off the filler I added to get a clean image in the original format.
What's the simplest (free) program I can use to do this?