thumbnails Options

[Smarmosaur]

ok, i'm pretty sure this has been asked somewhere before, but here goes.

i want to be able to click a thumbnail on my website and open the graphic in a new window, that automatically resizes for the image. it would also be nice to use that new window if another thumbnail is clicked, so that the new image replaces the old one, and that auto resizes.

i have no clue what type of coding or whatever is used to do this. i want something simple, because i'm going to have dozens of these floating around my site. i'm just simply asking for somebody to point me in the right direction.
any help appreciated! :]