Modifying the default post – gallery template

I’m attempting to put together a site that functions much like wwtdd.com or thesuperficial.com. I would like to show multiple pictures with every post, and would like each post to feature 1 large picture, text, then multiple thumbnail images. I’m having trouble figuring out the best way of accomplishing this.

From what I’ve seen, I really like the way WordPress handles the uploading/managing of multiple pictures to a post (so I don’t necessarily want to use a plugin like NextGen), but I can’t seem to figure out how I would go about specifying which one of the images I’d like to be the “big” one at the beginning of the post. I’m imagining there’s a template somewhere I can modify to do what I’d like, but I can’t figure out where! What file dictates gallery posts, or is there a plugin that would facilitate what I’m looking for better than a template hack (maybe I don’t fully appreciate what NextGen can do)? Thanks…