[Linuxcommand-discuss] Resizing multiple images using Terminal

[Ania <pi...@gm...>]

Hello,
this is my first experience with Terminal so I am a complete newbie :) 
but I need it to process multiple images which are to be used later in a 
video.
The first thing that I wanted to do was to minify the images of one 
sequence. I keep images for each sequence in separate folders. To make 
sure that the scaling will be proportional, I divided the original size 
by four and scaled first one of them with Gimp. These are the image 
dimensions:
Width: 2592 Height: 3872
And four times smaller:
Width: 648 Height: 968
The target image should have height 480 because the video will be 
720x480, but I didn't know how to write a command to change only height 
and adjust width proportionally, so to avoid distorting them I separated 
vertical images from horizontal in different folders. When the images 
are close to the target size I want to crop them (not sure how to do it 
yet).
I wrote this command line:
mogrify -convert -size 648x968 *.jpg -resize 648x968
And the answer was:
mogrify: unrecognized option `-convert' @ 
error/mogrify.c/MogrifyImageCommand/4560.

What am I doing wrong?