Interaction for Thumbnail/Large-Image Problem

Hi - I'm using Flash MX 2004 Professional on a PC.

I'm putting together my first Flash site for a wedding photographer. A site of 5 pages (Home, About, Contact, a Black & White Gallery and a Colour Gallery).

Now, I'm not sure what I need to know is too complex to explain, or extremely simple, but it's not in my ...for Dummies book, nor can I find it in the Help menu. Here 'tis:
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Each gallery has 12 thumbnails on the left side of the page, with a blank space on the right for a larger version of the thumbs to appear for the viewer to have a good look at the work when a thumbnail is clicked. I want the user to be able to click on a thumbnail which will fade a picture in. Then upon clicking on _any_ of the other thumbnails I want the existing larger image on the right to fade out and the next-thumb-that-was-clicked's relative larger image to fade in (I'm prayin' that makes sense to someone!)
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To me it seems like a very basic task for a Flash designer to be able to take care of, but I feel like I've tried everything that seems logical to me, but Flash is such a great app, the REAL solution will jump at me like proper logic, I just can't wait to find out! Being so new to Flash, an understanding on how I can achieve this will take my abilities from a mere animator-using-Flash to entire-site-builder-using-Flash. I can't wait to be able to do it. I would really appreciate some advice on how to achieve this. The site looks beautiful and I really am just a few hours off getting it launched.

Thank you in advance,