function updateFramePicture( table, number ) {
	
	var imageContainer=document.getElementById("ornamentPreview");
	imageContainer.src = 'images/products/prints/'+table+'/'+number+'_thumb.jpg';
	
}
<select name="previewList">
.
.
.
<option value="dogs_SHEL"onClick="updateFramePicture('dogs', 'SHEL');">Sheltie</option>
.
.
</select>


<td><img id="ornamentPreview" border="0" src="images/products/prints/dogs/AUS_thumb.jpg" alt="" /></td>