If you can use a FTP client and you have access to phpmyadmin (no offense meant, I can't know), this is fairly easy to achieve.
Copy the images to a folder on your disk, and reupload them to /wp-content/uploads/subfoldernameofyourchoice
(me, starting when wordpress gallery features were less cool, I've stuck to having my own /images/ subfolders not managed by wordpress at all, but to each his own)
And then, following Lorelle's guide : http://lorelle.wordpress.com/2005/12/01/search-and-replace-in-wordpress-mysql-database/
, update the content of your posts so that hyperlinks with /wp-content/themes/directorypress/thumbs/ are replaced with /wp-content/uploads/subfoldernameofyourchoice
I imagine there must be more "elegant" solutions in which the images are managed by wordpress as part of its inner gallery system, but that, I don't know how to do it ^^