well, it's possible ... it's not a feature of fancybox but there is a
workaround.
you need to override the css properties of the fancybox this way:
just AFTER you load the fancybox style sheet (and any other style
sheet)
<link rel="stylesheet" type="text/css" href="jquery.fancybox.css"
media="screen">
add the following code:
<style type="text/css">
div#fancy_inner {background-color: transparent; border: 0;}
div#fancy_outer {background-color: transparent;}
div.fancy_bg_n,
div.fancy_bg_ne,
div.fancy_bg_e,
div.fancy_bg_se,
div.fancy_bg_s,
div.fancy_bg_sw,
div.fancy_bg_w,
div.fancy_bg_nw {background-image: url();}
</style>
eventually you may want to set 'overlayShow' to false in your script
so your images will fade in and out from your web page without any
background. Personally I don't like it (I prefer fancybox as it is)
but it works.
You may also want to try changing the 'overlayOpacity' until you get
your preferred effect.
have fun!