Add padding around image

This will add a colour around an image, and convert any transparency to that colour as well. This adds 300px to each side, adjust -border to change it. It uses GraphicsMagick. Adjust the colour parameters and input/output filenames as needed.

gm convert input.png -bordercolor '#14181c' -border 300x300 -bordercolor black -background '#14181c' -flatten output.png