The quickest and easiest way to do this would be to have a placeholder image the same size as the graphic one, such as an image that says WARNING! GRAPHIC! etc., and surround it with an empty link. This way, you have access to the JavaScript onMouseOver event, so you can show the graphic picture when the mouse scrolls over the picture.

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First and foremost, this belongs in the Javascript/Ajax/DHTML section.
There are several things you may want to try:
First of all, the options in a select DOM aren't an array - if you want to change them, you'll have to remove them, then add the new ones.
You're missing some semicolons, and have some spelling errors (doucment, mychocie instead of myChoice).
Try reading up on the add() and remove() methods of the select...