pnmmargin
Add a border to a PNM image. More information: https://netpbm.sourceforge.net/doc/pnmmargin.html.
- Add a border of the specified size to a PNM image:
pnmmargin {{size}} {{path/to/image.pnm}} > {{path/to/output.pnm}}
- Specify the color of the border:
pnmmargin -color {{color}} {{size}} {{path/to/image.pnm}} > {{path/to/output.pnm}}