The images uploaded are in /wp-content/uploads/ and usually organised my year and month directories. Keep in mind that the image sizes that WordPress generates are also in here so each image will have a few versions of different sizes.
If you have SSH access you could probably run a find/copy to strip out the WordPress generated ones and copy the originals to another directory to be archived. When I say strip out, I mean from the copy command - not from the uploads directory.
I might be other thinking it, there might be a plugin that does this all for you.