i was wondering how I would use AS3 to create a dynamically loading photo gallery with a horizontal scrolling thumbnail bar at the bottom and have it so that when you click on one of the thumbnails, a larger version appears above the thumbnail bar.

Flash is not best way to create photo gallery. Too many limitations. There is impossible to save image to your local PC from Flash gallery.

What would be the best way to create a photo gallery then?

I would use javascript and css, perhaps just css... secret is to put the image in a div and set either height or width to 100% and the other to auto when you resize the div the image resizes and the aspect ratio is maintained by auto setting. Set overflow to display and only change the parameter of the div that is set as 100% for image.

.zoom{
 overflow: visible;
}

.resize{
width: 100%;
height: auto;
}

<div class = "zoom"><img class = "resize" src = "myimage.png"></div>

Scrolling is accomplished buy putting a div in a div

.display{
overflow: hidden;
}

<div class = "display"><div class = "slide"></div></div>

repositioning the "slide" div will result in different parts of it being displayed in "display" div.

Thanks Sleeping Troll. I'll try that out.