Q:I have multiple users posting my blog. I need to know how to make all image attachments they upload link to the original image (instead of permalink) by default.

A: There is a hidden options page in WordPress under http://yoursite/wp-admin/options.php
On that endless list of undocumented options you can set a value for "image_default_link_type". You may want to set it to "file" (without the quotes) to link all your images by default to the original image.

http://wordpress.org/support/topic/make-image-attachments-default-link-to-original-image
other info about galleries: http://en.support.wordpress.com/images/gallery/