My sister got engaged recently, and I had a lot of high resolution pics, which I wanted to upload. I needed to resize these images, in order to make them small and upload them.
I found out about a command called "mogrify". All I had to do was:
1) Create a separate folder, and backup the high-res pics, as they will be overwritten.
2) Run the command mogrify -geometry 640x425 ./*.* in the directory that I had to resize the images.
In a couple of minutes, all the images will be resized !!
Mogrify can do a lot more than just resize images. Run mogrify -help for details. :)