Batch resize images into new folder using ImageMagick

convert is designed to handle a single input file as far as I can tell, although I have to admit I don't understand the output you're getting. mogrify is better suited for batch processing in the following style:

mogrify -path ../dsc_small -define jpeg:extent=2MB dsc_big/*

But honestly I consider it dangerous for general usage (it'll overwrite the original images if you forget that -path) so I always use convert coupled with a for loop for this:

for file in dsc_big/*; do convert $file -define jpeg:extent=2MB dsc_small/`basename $file`; done

The basename call isn't necessary if you're processing files in the current directory.

This was the command which helped me after a long try.I wanted to make same sized thumbnails from a big list of large images which have variable width and height . It was for creating a gallery page.

convert -define jpeg:size=250x200 *.jpg  -thumbnail 250x200^ -gravity center -extent 250x200  crop/thumbnail-%d.jpeg

I got re-sized thumbnails which all having same width and height. :) thanks to ImageMagick.

Here's a solution without using for loops on the console

convert *.jpeg -define jpeg:extent=2MB -set filename:f '../dsc_small/%t_small.%e' +adjoin '%[filename:f]'