swap image with html form drop down list? HELP!!

hi, i have a working form which is used to select the colour of an item before it is added to my shopping basket, but what I want to do is allow the user to preview the item before adding it to the shopping cart. The simplest way i can think of is a javascript swap image script when the user selects a colour from the drop down list of colours. Anyone know how to do this? All i can come up with is the markup below but this means that the image is only swapped when list is changed and cannot allow me to display a preview image for each colour.

<select name=ADDITIONALINFO onChange="MM_swapImage('preview','','images/previewtees/dmlogoteepred.gif',1)"><option value="red">red</option><option value="blue">blue</option><option value="orange">orange</option><option value="green">green</option><option value="white">white</option></select>

Any help would be great.