You have to be able to sign into your actual webhosting account to get to that folder. The DNN host account isn't the same thing.

If the module will let you change the image paths to anything, upload the new images to your portal via Admin, File Manager and then create paths that point to them there as follows http://domain.com/portals/(portal#)/image.gif 

You can find your portal# grayed out in the portal's Admin Settings page, under Home Directory. It is usually a number, but may also be a name.

Otherwise, you'll have to find out how to get into your webhosting account, or find your system admin if you're using this in a business somewhere.

Rob