I am using this function to re-size my thumbnail , add_image_size( 'portfoliolarg', 120, 120, true); the image is corp and resize I just want to resize my Image
First of all:
add_image_size() is to ADD a new image size into your WordPress installation.
Now, as you said:
I just want to resize my Image
You need to enter into your wp-admin panel. Then Settings » Media, and change the thumbnail size there.
Just change the thumbnail size there. Now use the plugin: Regenerate Thumbnails by Viper007Bond to make your new image sizes.
So, after then, if you use:
the_post_thumbnail( 'thumbnail' );
it will echo the new thumbnail image (120 x 120).
WARNING: Changing image sizes and regenerating new image sizes, doesn't delete previously created sizes, just add new sized images into the server space. So the process is somewhat space-killing theory, if not smartly handled.