Image A behind Image B

I dont know how complicated this is but I've seen it done on other websites. How difficult is it to move Image A behind Image B? Like the middle of Image B is transparent and I want to see Image A behind it... its gotta be a style sheet or something. Has anybody else encountered this issue?

Thanks

I remeber seeing somthing like this with text overlapping
I think it's somthing to do with Layers and Z-order

Does this help at all?
http://www.meyerweb.com/eric/css/edg...iral/demo.html

CSS 3 has opacity proposed, IIRC.