Sounds like you might want to try one of the automated and free flash templates in BridgeCS4. If you own CS3 and have Photoshop CS3, look under File, Automate and Web Gallery. In CS4, they moved that functionality to Bridge.
No coding needed, just pick a template, choose your colors, pick your images and boom, you're done.
Otherwise, coding a gallery can be as simple as placing thumbs on stage with an invisible button on each that loads main image into an empty movieclip or as complicated as an xml driven gallery.