I downloaded the code for Nautilus Image Converter and been through it configuration file and I dont think that this is possible from configuration file. But, if you open nautilus-image-resizer.c and take a look at line 320, you will see the following code:

gchar *argv[6];

argv[0] = "/usr/bin/convert";

argv[1] = filename;

argv[2] = "-resize";

argv[3] = priv->size;

argv[4] = new_filename;

argv[5] = NULL;

as you see, this is where the arguments for Imagemagick convert utility are being created. I think if you change this to following, it should solve your problem:

gchar *argv[6];

argv[0] = "/usr/bin/convert";

argv[1] = filename;

argv[2] = "-resize";

argv[3] = priv->size;

argv[4] = "-strip"

argv[5] = new_filename;

DISCLAIMER: I havent tried this. This answer is solely based on my limited understanding of the Nautilus Image Converter code. So I would recommend to backup your images before using the modified version.