dhoomketu.net.in

Canon A-series cameras produce photos which are of aspect ratio 4:3. To
convert this to 8"x12" @ 300dpi (aspect ratio 3:2, 3600 x 2400),
- create a white jpeg of 3600x2400 in the current directory, called
white.jpg

convert IMG_1234.JPG -resize 3600x2400 -size 3600x2400 xc:white +swap
-gravity center -composite resized/IMG_1234.JPG

.. and save yourself 10 bucks. This will create white bars on the sides

to maintain aspect. If you're smart, you'll change the gravity so that
you get a large white bar on the left. This is useful to punch holes and
file the photos :)

For the Nikon D40, which produces 3008x2000 (almost 3:2, guess they
added the 8 to make it divisible by 16), crop instead.

convert dsc_1234.jpg -crop 3000x2000+4+0 -resize 3600x2400
resized/dsc_1234.jpg

Source: newsgroup