everything transparent…

I just wanna make the content area background a little bit of transparent…so I add the codes into the style.css like this

.box{

background: #FFF;
filter:alpha(opacity=90); /* Internet Explorer */
-moz-opacity:0.9; /* Mozilla 1.6 and below */
opacity: 0.9; /* newer Mozilla and CSS-3 */
}

but the images on the post also get transparent…so I add these codes

img {
filter:alpha(opacity=100); /* discarded */
-moz-opacity:1.0; /* discarded */
opacity: 1.0; /* discarded */
}

it doesnt work at all.I wanna make the post background transparent…but I dont wanna make the images transparent too.