Button..Button?

How do you get a button to activate when you change frames?

I have a series of thumbnail images. Once you click on an image, the playhead jumps to a frame on the timeline that contains a loader image gallery. The problem is with the particular loader image gallery I have you have to click on a thumbnail to activate the main_mc which displays the larger image.

Is there a way to automatically have a button activate once the playhead hits a certain frame?

Thanks