Very brief: Make object state of two frames: a small empty frame and full size picture.
Make the small picture a button that triggers the object state to full picture.
You can then add a close X button that reverses the object state.
sorry, but could you expand on that a little? apologies but I may need some step by step advice.
Go to Tv.Adobe and do a search for interactive and/or object states.
One other note: search for the topic with workflow export to SWF and replace in PDF.
If you don't want SWF, then only use buttons (no object states).
If you don't mind Dutch, take a look at a video at my blog about pop up in PDF etc: