I am not to sure what you are exactly after but think this may be it:
$imagepath1 = "c:/dir/......."
$imagepath2 = "c:/dir/......."
$imagepath3 = "c:/dir/......."
<SELECT name="dropdown1">
<OPTION value=" <?php $imagepath1?> "><?php echo "imagename1"?> </OPTION>
<OPTION value=" <?php $imagepath2?> "><?php echo "imagename2"?> </OPTION>
<OPTION value=" <?php $imagepath3?> "><?php echo "imagename3"?> </OPTION>
</SELECT><BR>
note that i have put the image name in php tags obviously you do not need to do this, unless you were using variables to identify the image name.
You could also you a 'for each' loop to cycle through each element in the dir. But don't worry about that if you do not know how to do that.
Is this what you meant?