Finally a flash question.....
What you want to do is in flash make a box over your poster in a different layer. Highlight the box, and hit F8... make it a button. Inside here, make a keyframe on the hitstate, and go to the first keyframe, and delete the content (the box you originally made. This will make an invisible box that will be clickable. Then you have to know ActionScript. use scriptassist if its version 8, it will do it all for you..
Mac G5 workstation with 4 gigs of ram, 4x250 gig drives in raid 5. High availability cluster server running centos for a server. 2xCanon GL2 cameras.