Why do you not want to produce the whole gallery in Flash? Is it a matter of the display size of each image or is it something else?
I'm absolutely terrible at html, and I can barely understand Actionscript as I'm a graphic designer more than a web designer. I am completely open to alternatives, but the effect I want still remains in that when one of the menu buttons are pressed, a group of images that fit that category slide into view from offscreen and are interactive so they can be enlarged.
I thought of using LightBox2, but all I can find is that it works in html only. If that's the case, I can't input my thumbnails into Flash, have them move onto screen, and then have them trigger the Lightbox when clicked.
But again, if I can understand the process, I'm open to alternatives.
Yes, that kind of thumbnail - large image viewer can be created in Flash. There are literally hundreds of variations available. As an example, I have six versions available on my personal site: http://robdillon.com/flash.html . You can freely download the source files for each of these and adapt one of them if you like.
Here's a link to good tutorial on a different style of gallery: http://www.flashmagazine.com/tutorials/detail/as3_photo_gallery/
As you'll each one follows a similar design structure. You may be able to put together something that meets your needs based on parts of several of these examples.