Bu şekilde istiyorsun o zaman:

$text = '
background: url(img/back.gif)
background: url(xxx/b.gif)
background: url(img/c.gif)
background: url("resim/a.gif")
background: url(http://adres.com/back.gif)
background: url("http://adres.com/back.gif")
background: url(\'http://adres.com/back.gif\')'; 


$yeni = preg_replace('/url\((\"|\')?([^http\:\/\/\/]*)\//smi','url($1../$2/',$text);
echo $yeni;
Çıktısı:

background: url(../img/back.gif)
background: url(../xxx/b.gif)
background: url(../img/c.gif)
background: url("../resim/a.gif")
background: url(http://adres.com/back.gif)
background: url("http://adres.com/back.gif")
background: url('http://adres.com/back.gif')