Adding RGBA

Hello, I'd like to add RGBA to my website www.luismario.com, I have added a background image but it is no showing. I have tried the RGBA samples you have posted in the forums but they don't seem to work for me. This is how my CSS looks like in case it is interfering: h1 {text-align: center;} #branding ul.menu a { text-transform: uppercase; } .tinynav {text-transform:uppercase; } #site-generator .copyright {margin: 0 auto; text-align: center; width: 100%; } .hentry, .no-results, #author-info, #disqus_thread, #content .error404, #colophon, #site-generator { border: none; } Thank you so much in advance