var s = '[2003][image/1.gif]';
    var re = /\[[^\]]+([^\/]+\.gif)\]/;
    s = s.replace(re, '<img src="images/$1">');
    alert(s)

------解决方案--------------------
var str = '[2003][image/1.gif][2003][image/1.png]';
var re = /\[([^\]]+\.gif)\]/g;
alert(str.replace(re,'<img src="$1">'))

------解决方案--------------------
JScript code

var str = '[2003][image/1.gif][2003][image/1.png]';
var re = /\[(\w+/[^.]+\.gif)\]/gi;
alert(str.replace(re,'<img src="$1">'))