I agree with pmw. The only thing I would do differently is the "hide/show" images. I think it would be easier instead of having to hide and show each individual image, you instead replace the image onchange with accordance to the option value.
For instance, to reference the select form as an object you would do:
var mylist = document.getElementById("mylist")
This gives you access to all it's properties and functions (or methods) in the my list variable.
There are short cut ways of accessing which option is currently selected. If I were doing this I'd make the image src names consistent with the option values. For instance:
<option value="Option">Option 1 </option>
var ext = ".gif"
Assuming the name of your first image is Option.gif, it will pop up accordingly. Basically just put the image name in the value of the option, access it, and add an extension.