<select name="mySelect" onChange="document.myImg.src = this.value" >
<option1>
.
.
.
</select>
<img name="myImg" />
make sure that the options are done like so:
<option value="%imgdir%\img.jpg" />img.jpg</option>
also, leaving the img tag without a default src draws a square with a red x in the top left corner. You might want to put in either the first img in the select box or a one white pixel bitmap...