Adjusting Background Size for Image?

Whether or not this is easy to code, I’ve seen it used very widely. You simply click on an image on a website and it’s loaded in another spot (via movie clips I assume). The spot that it’s loaded in could be a box that resizes itself to accommodate that image perfectly. How is this done? If my description is unclear, just ask and I should be able to find an example easily enough. Also, I’d like to know how to make a browser pop-up window that resizes itself to fit an image (shown here under gallery) and a flash pop-up window that resizes itself. Thanks for any and all help!