When your image is clicked, you could recreate all the options of your select. Put the 5 you want at the top, and make sure the top one is selected. Then pad the select box with some empty options, then add in the rest. The advantage of using this method is that it's still an HTML box and it's really easy - you're just modifying the innerHTML of a select box, after all! The disadvantages are that 1)Different browsers have different scroll-bar thresh-holds, but the distinction should still be clear to the user, and 2)depending on how many hundreds of options you have it could be quite slow. You could get around this by pre-generating the code for the options when the image is clicked.
Or you could just use a script to simulate a select, of course! You'll find several if you google for them, but the basic idea is to have a popup scrollable div layer underneath your "field".