When I change the Opacity of a Div, shouldn't the images inside the Div change Opacity also ?
Depends on the browser, so it's best to believe the answer is no. If you're doing this with jQuery, you can easily change it by doing:
$('div, div > img')
or, if you want it for all elements:
$('div, div > *')
If, on the other hand, you're not doing it with jQuery, please start doing so