Defining variable through thumbnail's instance name

This is probably one of the simplest things to do in as3, but let me tell you that when I say I am a newbie at this I mean I am a diaper-wearing green horn, but we all have to start somewhere I guess.

The Question:
I have 20 thumbnails and 20 full images that correspond to these thumbnails. I want to be able to load the corresponding full size image from each thumbnail so that thumb_1.jpg will load fullsize_1.jpg, and thumb_2.jpg load fullsize_1.jpg, so forth and so on.

I only want to make one button do this for every single image, which I imagine is possible through as3 if I can make thumb_“imageNumber”.jpg set the full size image name fullsize_“imageNumber”

Again this is probably one of the most basic questions ever posted to this forum and I apologize gravely if it has been brought up on previous threads. If so, please direct me to the thread where this issue is addressed and I will be more than grateful.

Thank you in advance.