Welcome to the forum. Flash allows for many different ways to accomplish the same goal. Something like this could be achieved entirely with actionscript, but to keep it simple, my suggestion would be instead of seperate movieclips for each .... use a single movieclip with each image and description on a different frame and use a rollOver event to send the movieclip timeline to the correct frame...
Movieclip named "imageDescription"..... button code for rollover...
hope it helps
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