Have the Background color be an image?

I have my background color set to transparent in the CSS Stlyesheet, but I've been hoping to replace the color all together with a partially opaque image. I don't really have any code to provide as a starting point, other than the code I'm using to get it all transparent.
I'm using PHPBB3, and any help would be GREATLY appreciated!

Code:

td.bodyline {
background: rgba(255,255,255,-510);
}
td.bodyline{
border: 0px !important;
}

And here's the forum link: http://terratempusforum.forumotion.com/

Thank you! I'll this a try later today!
Edit:
Would there be anyway to set opacity to that? I'd like to have it somewhat transparent, but still visible.
Edit2:
Well now I've got it opaque.. but it's EVERYTHING, not just the background image. Is there anyway to just make the background image opaque? Here's the code that i've created:

Code:

td.bodyline {
background-image: url('http://www.ipad-wallpapers.us/bgs/text-on-a-paper-ipad-background.jpg');
opacity:0.8;
background-repeat: no-repeat;
}

Anyone?

Rideem3 wrote:No, you'd have to edit the image itself since it wraps around other elements which will make everything opaque.

I guess i'll get right to it, thank you! Do you happen to know the size of the max image width/height?

Edit:
Ah I'll guess and check. Thanks for the help!