ImageMagick

Creating Thumbnails

Creating a thumbnail that keeps the aspect ratio and adds a border when necessary.

file_name=image
file_extn=jpg
convert -define jpeg:size=150x150 "${file_name}.${file_extn}" \
    -resize 150x150 \
    -background white -gravity center \
    -extent 150x150 "${file_name}-150x150.${file_extn}"

Or, relying on defaults in ImageMagick:

convert ${file_name}.${file_extn} -scale 150x150 -gravity center \
    -extent 150x150 ${file_name}-150x150.${file_extn}