è un po lunghetto... ma funziona:

<script>
<!--
// Startup variables
var imageTag = false;
var theSelection = false;

// Check for Browser & Platform for PC & IE specific bits
var clientPC = navigator.userAgent.toLowerCase(); // Get client info
var clientVer = parseInt(navigator.appVersion); // Get browser version

var is_ie = ((clientPC.indexOf("msie") != -1) && (clientPC.indexOf("opera") == -1));
var is_nav = ((clientPC.indexOf('mozilla')!=-1) && (clientPC.indexOf('spoofer')==-1)
&& (clientPC.indexOf('compatible') == -1) && (clientPC.indexOf('opera')==-1)
&& (clientPC.indexOf('webtv')==-1) && (clientPC.indexOf('hotjava')==-1));
var is_moz = 0;

var is_win = ((clientPC.indexOf("win")!=-1) || (clientPC.indexOf("16bit") != -1));
var is_mac = (clientPC.indexOf("mac")!=-1);


// Define the bbCode tags
bbcode = new Array();
bbtags = new Array('[img]','[/img]','','','','','
','
','
codice:
','
','
  • ','
','
  1. ','
','','','','');
imageTag = false;

// Shows the help messages in the helpline window
function helpline(help) {
document.FormPost.helpbox.value = eval(help + "_help");
}


// Replacement for arrayname.length property
function getarraysize(thearray) {
for (i = 0; i < thearray.length; i++) {
if ((thearray[i] == "undefined") || (thearray[i] == "") || (thearray[i] == null))
return i;
}
return thearray.length;
}

// Replacement for arrayname.push(value) not implemented in IE until version 5.5
// Appends element to the array
function arraypush(thearray,value) {
thearray[ getarraysize(thearray) ] = value;
}

// Replacement for arrayname.pop() not implemented in IE until version 5.5
// Removes and returns the last element of an array
function arraypop(thearray) {
thearraysize = getarraysize(thearray);
retval = thearray[thearraysize - 1];
delete thearray[thearraysize - 1];
return retval;
}


function bbstyle(bbnumber) {
var txtarea = document.FormPost.Post;

txtarea.focus();
donotinsert = false;
theSelection = false;
bblast = 0;

if (bbnumber == -1) { // Close all open tags & default button names
while (bbcode[0]) {
butnumber = arraypop(bbcode) - 1;
txtarea.value += bbtags[butnumber + 1];
buttext = eval('document.FormPost.addbbcode' + butnumber + '.value');
eval('document.FormPost.addbbcode' + butnumber + '.value ="' + buttext.substr(0,(buttext.length - 1)) + '"');
}
imageTag = false; // All tags are closed including image tags
txtarea.focus();
return;
}

if ((clientVer >= 4) && is_ie && is_win)
{
theSelection = document.selection.createRange().text; // Get text selection

if (theSelection) {
// Add tags around selection
document.selection.createRange().text = bbtags[bbnumber] + theSelection + bbtags[bbnumber+1];
txtarea.focus();
theSelection = '';
return;
}
}
else if (txtarea.selectionEnd && (txtarea.selectionEnd - txtarea.selectionStart > 0))
{
mozWrap(txtarea, bbtags[bbnumber], bbtags[bbnumber+1]);
return;
}

// Find last occurance of an open tag the same as the one just clicked
for (i = 0; i < bbcode.length; i++) {
if (bbcode[i] == bbnumber+1) {
bblast = i;
donotinsert = true;
}
}

if (donotinsert) { // Close all open tags up to the one just clicked & default button names
while (bbcode[bblast]) {
butnumber = arraypop(bbcode) - 1;
txtarea.value += bbtags[butnumber + 1];
buttext = eval('document.FormPost.addbbcode' + butnumber + '.value');
eval('document.FormPost.addbbcode' + butnumber + '.value ="' + buttext.substr(0,(buttext.length - 1)) + '"');
imageTag = false;
}
txtarea.focus();
return;
} else { // Open tags

if (imageTag && (bbnumber != 14)) { // Close image tag before adding another
txtarea.value += bbtags[15];
lastValue = arraypop(bbcode) - 1; // Remove the close image tag from the list
document.FormPost.addbbcode14.value = "Img"; // Return button back to normal state
imageTag = false;
}

// Open tag
txtarea.value += bbtags[bbnumber];
if ((bbnumber == 14) && (imageTag == false)) imageTag = 1; // Check to stop additional tags after an unclosed image tag
arraypush(bbcode,bbnumber+1);
eval('document.FormPost.addbbcode'+bbnumber+'.valu e += "*"');
txtarea.focus();
return;
}
storeCaret(txtarea);
}
-->
</script>

e poi:

<form name="FormPost" id="FormPost" method="post" action="pagina.php">
<input class="button" onClick="bbstyle(0)" type="button" value=" IMG ">

</form>