Ok here is a working example with functions and all.
HTML Code:
<html>
<head>
<title>Test</title>
<script type="text/javascript">
function GetSelectedValue(select) {
var id = select.options[select.selectedIndex].getAttribute('name');
AddToInput(document.forms[0].textBox1, 'string_left ' + id + ' string_right');
}
function AddToInput(input, value) {
input.value = value;
}
</script>
</head>
<body>
<form action="" method="get">
<p>
<label>Select image</label>
<select name="TestSelect">
<option name="1">Image 01</option>
<option name="2">Image 02</option>
<option name="3">Image 03</option>
</select><br /><br />
<label>Selected value</label>
<input type="text" name="textBox1" /><br /><br />
<input type="button" name="getValue" value="Get value" onClick="GetSelectedValue(document.forms[0].TestSelect)"; />
</p>
</form>
</body>
</html>