Hiya mate,
This is a problem I've had many times. My favourite way to solve it (because it gives most flexibility) is to add each new clip to an array as you create them.
So, I'll try a step by step:
1. Declare a new blank array at the top of your script:
var aImageReference:Array = new Array();
2. Each time you "loop" through your code that attaches the image to the thumb clip, push a reference to it on to the array. I'm not sure what you've called them but let's say it is "thumb_mc":
aImageReference.push(thumb_mc);
3. Now, whenever you want to access a clip, just use array access notation. So, if you want to change the alpha of the third image in the series:
aImageReference[2]._alpha = 50;
NB. Arrays are zero based so the third clip is number 2 (because the first one is zero!).
Finally, just be careful of one thing. If your movie performs this task over and over (e.g. it attaches different clips to different instances on different "pages" of your site), be sure to clear the array each time before you start pushing back onto it:
aImageReference = new Array();
Any probs, let me know!