How exactly does the gallery work in 2.5 ?

Recently one of my friend and client asked me to edit the look of the his new gallery view in the new version of wp 2.5. Now he gave me a image.php to put inside theme folder and said it makes the gallery work.

Although I have made few themes for wordpress and am pretty good with CSS, all this gallery is going above my head. Is there any doc regarding the gallery or how to add it ? I searched a lot and all lead to nothing.

I also came upon this codex entry but it just states how to add [gallery] inside the post. but nothing about how to add images or how to sort etc.

Any help if appreciated. Thanks.