There is a way to use the drop-image option to change the main image, it requires a javascript and template change...5-10 mins of work (more or less)
Edit:
assets/templates/common/js/listing.js
(Make a backup first, also if you want to retain the normal drop-down image behavior then simply make a copy of the file and name it listing2.js)
Edit the file, find the line 183, should say:
HTML Code:
// change main image
//SetSrc(what,"large",GetValue(what,"image_" + what.elements[i].options[what.elements[i].selectedIndex].value));
Change to:
HTML Code:
// change main image
SetSrc(what,"large",GetValue(what,"image_" + what.elements[i].options[what.elements[i].selectedIndex].value));
(Basically remove the // from the front of sersrc)
Also remove the previous line:
HTML Code:
SetSrc(what,"img_" + fieldnamemod,GetValue(what,"image_" + what.elements[i].options[what.elements[i].selectedIndex].value));
(erase it from file).
Next, if you do want to keep both behaviors, instead of editing your listing_0.html, make a copy of the file, and name it listing_1.html, if you already have a _1, name it _2, (if you have a _2 already name it _3, etc.)
On the very top of listing_x, you'll see
HTML Code:
<script language="JavaScript" src="assets/templates/common/js/listing.js" type="text/JavaScript"></script>
If you did make a copy of listing.js, make sure you change this to listing2.js or whatever. If you didnt, then you can leave the file that line as is.
Now, still on listing_x.html file, search for:
HTML Code:
<img name="img_[oname]" src="assets/templates/common/images/spacer.gif">
Remove that line. (That's the place holder for the small image, which we don't need any more.)
I hope that's clear, if not, let me know

, post the link to an item after you are done!