<script type="text/javascript">
function sec()
{
document.getElementById("input").select();
}
</script>
<input size="110" type="text" id="input" value="[img=http://xxx.com/x.jpg]" onfocus="sec()" readonly="readonly" />

<input size="110" type="text" id="input" value="[img=http://xxx.com/x.jpg]" onfocus="select(this)" readonly="readonly" />